"Was Littleboy a Tallboy?" Topic

​Here's the thesis:

B17s could carry 20,000lbs. The British had 20,000lb Tallboy variants called Grand Slams. Could they deploy them as air-burst? British airburst 15,000lbers with incendiaries produced destruction with very similar effects to Hiroshima, including the mushroom cloud and the instant heat-radiation flash which left stone buildings standing but incinerated people. Incidentally, post-bomb-drop analysis at Hiroshima by Farrell and Morrison found little evidence of atomic radiation beyond the ground-norm. Then the record goes cold when the British were still experimenting, due to still-in-place secrecy ordinances, just before the Manhattan Project (where teams were highly compartmentalised) gets close to fruition. The main Hiroshima heat-wave destruction covered 2-miles, equivalent to 15,000lbs of TNT. B17s could carry the 20,000lb British bombs. The British perfected airburst with incendiaries and heat-waves.

There's little doubt that to get the effects at Hiroshima, Truman could have used a conventional bomb. But would the morale effect on Hirohito of TNT and petrol have been the same as making him fear the limitless potential of Oppenheimer's mysterious and futuristic "Destroyer of Worlds"?

Pretence would do it.

What about all the people who later died of radiation poisoning?

John the OFM
"What about all the people who later died of radiation poisoning?"

"Radiation poisoning" wasn't a general medical condition used in Japan until Americans in Japan defined it as such. The actual effects were skin lesions, hair loss (epilation), diarrhea, fever, bleeding gums, and hemorrhaging beneath the skin. The lung damage on its own would have long-term effects. All these symptoms are consistent with being caught in a heat-blast.

You have to remember, heat-waves are "radiation". Have a look at Farrell and Morrison's data.

There were increased levels of cancer in subsequent years and some increase in genetic malformations, Psychiatric effects (PTSD etc)- I have seen the data in the official publication 'the medical effects of nuclear war' BMA 1983, its well worth a read.

I'm always a bit cautious about medical data with claims of "increases in cases." Medicine is constantly expanding in technology and scientific understanding, especially in the 20th century. "Cancer" was still poorly understood, and the medical knowledge and capability of physicians varied widely around the world.
Thus an "increase" in the number of cases of cancer could be exactly that— or it could reflect greater testing, greater reporting to doctors by patients, better understanding of what was and wasn't cancer, greater technological ability to discover and diagnose cancer, and so forth. Thus, the "increase in cases" could be due to the after-effects of the bomb (and it's entirely reasonable to think that), or could be due to other factors not considered or simply due to "we're looking for it more, so we find more."

However, I seriously doubt that Littleboy was anything other than what it was reported to be— a true atomic bomb. At this point, subterfuge about that seems unnecessary.

What a fascinating suggestion, that I have never seen suggested before. Let us concentrate on blast and the fireball from an early A bomb. Radiation short term, let alone long term, consequences were truly not recognised in 1945. Clinicians had learnt that X ray exposure would destroy your hands, radium dial painters on watches lost their jaws to ORN, but that was all in its infancy.

Can incendiaries airburst? How many Lancasters actually carried that incredible bomb weight? B17s I think is a typo error, very first line above…B29s yes. The B17 bomb load was very poor for the unfortunates who had to fly to Schweinfurt unescorted in broad daylight, even with a Norden sight

I believe the novel was titled The Jesus Factor. Yes it was. Edwin Corley, 1970.

I'm claiming full bragging rights. I never read that one, but skimmed it when it was new in mass market paper back in Nixon't first term. And yes, Corley has a good reason why you'd fake an A-bomb. Don't let me spoil it.

Have a look at Farrell and Morrison's data.

🙄
Please be so kind as to provide a link.
Until then…..

Thank you Robert Piepenbrink for the book title and author. I immediately thought about it after reading the initial post. I bought the paperback book and read it while in college (55 years ago). The way the Hiroshima "A bomb" was duplicated in the book was far different than proposed here. I well remember the novel and also won't spoil the reason for faking an A bomb.

I read "The Jesus Factor" years ago too. It was of course a wor of fiction.

Now, about those 'moon landings'……

"It was of course a work of fiction."

And Cacadoress' post was…?

Tallboys and Grand Slams were "bunker busters", designed to explode underground. They were definitely not intended for air burst.

🙄
Please be so kind as to provide a link.
Until then…..

From the Radiation Injuries chapter of The Atomic Bombings of Hiroshima and Nagasaki by The Manhattan Engineer District, June 29, 1946 report:

"One of the most important tasks assigned to the mission which investigated the effects of the bombing was that of determining if the radiation effects were all due to the instantaneous discharges at the time of the explosion, or if people were being harmed in addition from persistent radioactivity. This question was investigated from two points of view. Direct measurements of persistent radioactivity were made at the time of the investigation. From these measurements, calculations were made of the graded radiation dosages, i.e., the total amount of radiation which could have been absorbed by any person. These calculations showed that the highest dosage which would have been received from persistent radioactivity at Hiroshima was between 6 and 25 roentgens of gamma radiation; the highest in the Nagasaki Area was between 30 and 110 roentgens of gamma radiation. The latter figure does not refer to the city itself, but to a localized area in the Nishiyama District. In interpreting these findings it must be understood that to get these dosages, one would have had to remain at the point of highest radioactivity for 6 weeks continuously, from the first hour after the bombing. It is apparent therefore that insofar as could be determined at Hiroshima and Nagasaki, the residual radiation alone could not have been detrimental to the health of persons entering and living in the bombed areas after the explosion.

The second approach to this question was to determine if any persons not in the city at the time of the explosion, but coming in immediately afterwards exhibited any symptoms or findings which might have been due to persistence induced radioactivity. By the time of the arrival of the Manhattan Engineer District group, several Japanese studies had been done on such persons. None of the persons examined in any of these studies showed any symptoms which could be attributed to radiation, and their actual blood cell counts were consistently within the normal range. Throughout the period of the Manhattan Engineer District investigation, Japanese doctors and patients were repeatedly requested to bring to them any patients who they thought might be examples of persons harmed from persistent radioactivity. No such subjects were found.

It was concluded therefore as a result of these findings and lack of findings, that although a measurable quantity of induced radioactivity was found, it had not been sufficient to cause any harm to persons living in the two cities after the bombings."

Chapter:
link

Report:
link

The point of the Tallboy and Grand Slam were to penetrate concrete suck as the reinforced roof of a U boat pen. The fact that they were used on Tirpitz was pretty much a use what you have scenario that worked well. The other 'off use' was to take out a viaduct by the seismic effects.

Later the US replicated them in great haste in 1991(?) by using left over eight inch gun barrel tubes as the basis of the casing to after after bunkers in the sand box. Later they developed more purpose built affairs.

The more destruxtive area weapons were the HC or high capacity bomb probably better known as the 'cookie'. I think it was about 4,000lb and designed to be set off immediately on mpact. I suspect that it could have been set as an airburst if effort were put into it.

The definite air burst fuzes were for the incindiereries. Developed by the RAF and taken up en masse by the XXth Air Force when they adopted the Bomber Command strataegies wholesale at wars end.

I know that B29s carried and practiced with the Tallbys. I do not think that they made it into the bombing campaign though. I have even seen photographs of a B29 carrying two 12.000lb Tallboys with one under each wing! They did use Tallboys in of all places Korea, guided by the Tarzon steerable tail fin.

As far as blast goes,,, not sure what you're getting at?
Close counts with horseshoes, hand grenades and oversized explosives.

Post war alterring the bomb wall for the target, changing the composition of the filler for the effect and using a fuze appropriate to the desired result are all part of the mix. But dead is still dead.

Oh yeah,,,, the Tall boy was 12,000 lbs and the Grand Slam was 22,000 lbs.

For a Lancaster to carry them they had to be str5ipped of anything un=necessary. The mid upper turret, the nose turret, the bomb doors removed and so much more.

For a B17, a payload of 20, 000lbs includes the entirety of the bombload, the fuel and ammo. Again could it have been done by a B17,,, theoretically yes, but that would have involved removing the turrets, guns, armour and superfluous crewmen and even then the airframe would have had to have been modified with great portions of the belly removed to opartially accomadate one. this might have made the structural integrity of the airframe at great risk.

Could a Halifax or a Liberator lifted a Tall Boy. Again theoretically yes, but for the same above reasons as the B17 and mostly that structural integrity issue No in the end. Worse yet the B24 could not have mounted it under eath due to such low ground clearance.

Never gave the Dominator a thought, but good t think about.

B29, yes and did it with gusto.

A B17 with a payload of 20,000lbs, however you come up with that figure, it would never have got off the ground.

That last posting, with all due respect, is a bit daft. Many a four engined bomber, if you remove the crew, the armour, the gun turrets, the undercarriage, the fuel tanks, the engines, etc could carry a contemporary A bomb…at least to the end of the runaway.

Easy, the original post mentioned 20, 000 lb and a B17.

I'ld be impressed if with 20,000lbs a B17 could be dragged never mind taxi under its' own power..

Let's just say that I am not impressed with the validity of the argument.

1. Are some asking if the ABomb should not have been used?
2. That a ground invasion should have taken place instead?
3. That fire bombing should have continued during the invasion as was happening prior?

That with options 2 and 3, Less civilians would have died? That in addition, well over a million allied military would have been killed and injured?

Somehow dying by incineration of conventional weapons, was any better than by an atomic blast?

Japanese civilians, men, women and even children had not fought with increasing fanaticism the closer the allies came to their homeland?

Fire bombing:

Tokyo, Japan: Between \(\text{80,000}\) and \(\text{100,000+}\) people died during a single nighttime raid (Operation Meetinghouse) on March 9-10, 1945.

Hamburg, Germany: Allied incendiary raids during Operation Gomorrah in July 1943 created a massive firestorm that killed approximately \(\text{42,600}\) civilians.

Dresden, Germany: The combined British and American raids in February 1945 caused a firestorm that resulted in up to \(\text{25,000}\) deaths.

Kobe, Japan: A major firebombing campaign on March 16-17, 1945, created large firestorms and destroyed much of the city, causing an estimated \(\text{8,800}\) to \(\text{15,000}\) fatalities.

Yokohama, Japan: A devastating U.S. incendiary raid on May 29, 1945, incinerated a large portion of the city and is estimated to have killed between \(\text{8,000}\) and \(\text{10,000}\) people

To the end of the original post question, No a display would not have done anything for a people who were considered fanatics and happy to die for the emperer.

It was only the aspect of complete destruction and an American invasion that got the top leadership oen to merely have talks with the Russians as intermediaries. The finality of the nuclear bombing, the mass destruction of the bombing campaign and the impending invasions by both the US as well as the Russians that forced their hand.

Could an invasion alone have done it?
Perhaps, but given the precedants of Okinawa and iwo jima, the casulties and the bitterness of the troops taking those casulties would have guaranteed that there were not enough Japanese left over to breed.

Seriously, it is even today that US troops are still being issued purple heart medals struck in 1945, with the invasion of Japan in mind.

Some have mentioned incindieries.
Incindieries work better as smaller bombs as 'starters'.
I think the regular bomb was composed of hundreds of four pound incindieries bomblets that would separate, spread out and start lots of fires.

Over Germany, it worked on historical towns where wood predominated, however for the most part they were combined with high capactiy or blast bombs that would tear buildings apart enough to create lots of rubble and debris that would act as kindling to start fires.

It worked quite effectively. Lots of little fires create bigger fires and so on , and so on, and so on, until you get a fire storm of hurricane proportions. The scientists working for the Army Air Corp studied the RAF campaign thouroughly. They even went to the extant of building Japanese villages and small town using Japanese construction methods and material to test out their bombing effects. Needless to say they reduced the high capacity or blast bomb ratio and increased their incindiary load outs and it worked only too well when the US adopted fire bombing.

A really large incndiary bomb does not give any advantage.

Anyone interested in this aspect, I can highly recommend the book, "Point of No Return, the story of the XXth Air Force" for some good explaining of the bombing campaign against Japan and the results of the switch in tactics.

More deaths were caused by the firebombing of Tokyo than the nuclear attacks. During the firebombing of Tokyo, 174 B-29s were used in the raid on Tokyo on February 25, 1945, and 334 B-29s participated in the more devastating raid on the night of March 9-10, 1945.

The key difference is that the two nuclear bombs that were dropped on Hiroshima and Nagasaki were delivered by one bomber each and one bomb each.

It has been estimated that an invasion of Japan would have cost the lives of 20 million Japanese. There was a severe food shortage in Japan. The 1945 rice crop failed.

The alternative to invasion was blockade and bombard as purposed by Admiral King and Hap Arnold. The result would have been catastrophic for Japan. The nuclear attacks actually saved Japanese and American lives. There were no good choices, you pick the one that will do the least harm.

By mid-1945, the daily death rate for noncombatants in Asia during WWII was 8,000–14,000 people per day, with the overwhelming majority being Chinese civilians This makes the Asia-Pacific theater one of the deadliest regions in human history. Everyday the war continued thousands died.

And King and Arnold's idea would have taken time. Not something most western country's civilians, politicians and media would have had the patience for. (As we have seen since WW2).

If the invasion had taken place, many of us would not have been born to comment here. Not speculation, just fact.

General Marshall was concern about the American people wanting a negotiated settlement, which was exactly what Japan was counting on. There were calls to cut war production in half and to start making consumer products. There was growing resistance to the redeployment of troops from the ETO to the PTO.

At the same time he was alarmed by intelligence that showed the Japanese moving a massive amount of troops into Kyushu. Now instead of a 3 to 1 odds, it was now even odds. He suggested to MacArthur that he may want invade further North.

The emperor was given a face saving way to end the war he wished to end. "They are using a nuke? Now I can surrender." Although some of the radicals in the Army almost couped the govt. Only the emperor stopped them.

To answer the OP. Little Boy was not a Tallboy. Different mission and different configuration.

I think someone dropped a few decimal points. The Hiroshima bomb was the equivalent of 15-20,000 TONS of TNT, not POUNDS.

+1 Scott.
The Math ain't mathing.

I think someone dropped a few decimal points. The Hiroshima bomb was the equivalent of 15-20,000 TONS of TNT, not POUNDS.

Yes indeed. The original post understates the power of the Little Boy bomb by a factor of about 2,000 X.

I read the whole thread to this point wondering who would spot that little gem. Thank you, Scott!

Once someone casually makes a statement in support of a position that is off by more than a couple orders of magnitude, I start to think that one of two cases are likely to be true:
a) This person doesn't know what he / she is talking about;
- or -
b) This person is deliberately trying to mislead.

Which shall we assume in this case?

As to the question of the bombs and bombers …

The Tallboy and Grandslam bombs were designed for deep penetration. Barnes Wallis (inventor) referred to them as "earthquake" bombs. They were streamlined to achieve supersonic speeds as they fell, and were as heavy as they were not because they carried such large explosive payloads, but because they had heavy thick metal casings to withstand impact. Compared to other bombs, they had relatively now % of HE content, at about 40% of their weight. By comparison, the British "blockbuster" bombs, the next largest bombs used in WW2 by the RAF, which were available in 3,000 and 4,000lb variants, had HE filler content at about 75% of their weight. That's what you build if you want a big BANG!

A supersonic penetrator with low HE content is just about the worst possible choice for an air-burst bomb. You won't make a very big bang for all your trouble, and all the fragments you generate (about 14,000lbs of fragments with a Grandslam) will start with a vertical velocity equal to the bombs' supersonic fall speed. You'll be lucky if you create a danger zone more than a couple dozen meters in diameter.

BTW, I don't know what the reference to a British air-burst 15,000lb bomb might be referring to -- anyone ever heard of such a weapon in RAF use in WW2?

Even IF a B-17 could lift 20,000lbs of payload (maybe in the shortest-distance ferry flight?), it most certainly could not lift a tallboy or grandslam bomb.

The Lancaster was the bomber for the job not just due to it's lifting capacity, but also do to the rather unique structure of the plane. It was a high-shouldered wing design, with the wing-spars crossing the fuselage above the area where the bomb bay was located. So it had a single layer, long, uninterrupted bomb bay. Bulge the doors a bit and you could fit a tallboy. Remove the doors, cut down the back of the fuselage a bit, and you could slide a grandslam beneath it.

The B-17 had a comparatively small and tall bomb bay. It was a low-wing design, with wingspars running through the fuselage low, reducing the length of the bomb bay. The bomb bay was only as long as a common 1,000lb general purpose bomb, with a bit of room to spare. Even the Navy's single-engined Avenger had a longer bomb bay. To carry more than 2 bombs (left and right), the B-17 carried them stacked in vertical racks going up inside the bomb bay. There was no place to put a longer bomb, or an out-sized bomb of any sort. Unless it was shaped like a tall box.

Or so I understand. Willing to learn if I am mistaken … from someone who knows what they are talking about.

-Mark
(aka: Mk 1)

There are those who will believe anything that contradicts what they have been told, just because.
This thread is prime proof.

Troopwoo
"Over Germany, it worked on historical towns where wood predominated"

Hiroshima was a mainly wooden-built city.

deadhead
"A B17 with a payload of 20,000lbs, however you come up with that figure, it would never have got off the ground"

Hiroshima effects diameter required a 15,000lb bomb.

Herkybird
"There were increased levels of cancer in subsequent years"
A massive shock to the immune system and the breathing in of carcinogens would do that.

Parzival
"However, I seriously doubt that Littleboy was anything other than what it was reported to be— a true atomic bomb. At this point, subterfuge about that seems unnecessary".

The $2 USD billion supposedly spent on the Manhattan Project during a war (about $33 USD billion to $38 USD billion today), when there was a conventional bomb that did the same job, wouldn't have struck Truman as somewhat "unnecessary"?

"Few know what they have been producing".

What was important, was not whether Littleboy was or was not an atomic bomb, but how Hirohito could be convinced that Hiroshima was just a taster of almost unlimited destruction to come. This is where Truman's use of hyperbole for Hirohito's benefit is evident: exaggerating the power of Littleboy first by 25% when it had a much lesser effect: just 15,000lbs of TNT. Aside from the explicit reference to their bombs, Truman also emphasises the British contribution. Then, despite saying Littleboy had "two thousand times" what he calls "the blast-power" of a Grand Slam (it had less), he then says, strangely enough, that the British "Grand Slam" "is the largest bomb ever yet used in the history of warfare".

From Truman's statement after Hiroshima:

"Sixteen hours ago an American airplane dropped one bomb on Hiroshima, an important Japanese Army base. That bomb had more power than 20,000 tons of T.N.T. It had more than two thousand times the blast power of the British "Grand Slam" which is the largest bomb ever yet used in the history of warfare.

"….With this bomb we have now added a new and revolutionary increase in destruction to supplement the growing power of our armed forces. In their present form these bombs are now in production and even more powerful forms are in development.

"It is an atomic bomb. It is a harnessing of the basic power of the universe. The force from which the sun draws its power has been loosed against those who brought war to the Far East.

"…Beginning in 1940, before Pearl Harbor, scientific knowledge useful in war was pooled between the United States and Great Britain, and many priceless helps to our victories have come from that arrangement. Under that general policy the research on the atomic bomb was begun. With American and British scientists working together we entered the race of discovery against the Germans.

"….Prime Minister Churchill and President Roosevelt agreed that it was wise to carry on the project here. We now have two great plants and many lesser works devoted to the production of atomic power. Employment during peak construction numbered 125,000 and over 65,000 individuals are even now engaged in operating the plants. Many have worked there for two and a half years. Few know what they have been producing. They see great quantities of material going in and they see nothing coming out of these plants, for the physical size of the explosive charge is exceedingly small. We have spent two billion dollars on the greatest scientific gamble in history-and won.

"…We are now prepared to obliterate more rapidly and completely every productive enterprise the Japanese have above ground in any city. We shall destroy their docks, their factories, and their communications. Let there be no mistake; we shall completely destroy Japan's power to make war.

"Although the workers at the sites have been making materials to be used in producing the greatest destructive force in history they have not themselves been in danger beyond that of many other occupations, for the utmost care has been taken of their safety.

"…The fact that we can release atomic energy ushers in a new era in man's understanding of nature's forces…."

About the Lancaster and the B-29. The only aircraft besides the B-29 that could be modified to carry Little Boy and Fat Man was the Lancaster. There was absolutely no way that any plane other than an American bomber was gong to carry them. The B-29 program was the most expensive project of WWII. The B-29 cost the taxpayers about 3 billion dollars, a billion more than the Manhattan Project. The B-29 came along at just the right time, serendipity.

…exaggerating the power of Littleboy first by 25% when it had a much lesser effect: just 15,000lbs of TNT.

… saying Littleboy had "two thousand times" what he calls "the blast-power" of a Grand Slam (it had less) …

Is it pointless to discuss such things with someone who spouts such preposterous conclusions?

Trusting in the value of reality, I shall try. But I do wonder if I am boxing with the shadow of a fool.

Let's put some hard numbers up, shall we?

_____

The Grand Slam bomb:

Explosive Filler: 4.1 tons TNT
Approximate Crater Diameter: 30-40m
(For a 4.1 tons HE blast done on the surface, which a supersonic penetrator would not normally do)

Diameter of Heavy Destruction: 50-100m
(Even stone or brick buildings are largely destroyed)

Diameter of Damage: a few hundred meters
(Lighter construction buildings may be rendered useless, heavily built structures survive but with damage)

These numbers are based on estimating the likely results if a Grand Slam was detonated from a static position at ground level. If dropped from an aircraft at altitude it would have detonated several dozens of meters underground. That's what it was made to do: penetrate.

_____

The largest non-penetrating British HE bomb was the Blockbuster (also often called "Cookie"). This was not designed for penetration, but for maximum damage to structures above ground.

HC4000 bomb:

Explosive filler: 3,200-3,600 lbs
(But not TNT. Still the TNT equivalence is often described as about 1.5 tons.)

Diameter of crater: Effectively nil. Not used for making holes in the ground.

Diameter of Destruction: 75m
(Lighter constructed buildings blown down. Heavily constructed walls may remain, but roofs blown down.)

Diameter of Damage: 150-300m
(Some structural damage, all but the lightest buildings are still standing.)

These numbers are based on assessments of the results of blockbuster bombs dropped on German cities.

_____

The largest conventional explosive bomb developed by the British or Americans to date is the post-war US GBU-43/B, a 21,600 lb HE bomb used by the US in the post cold-war conflicts in the Persian Gulf, and popularly referred to as the MOAB (Mother of All Bombs).

GBU-43/B:

Explosive Filler: Equivalent to 22.000 lbs of TNT
(A bit less in weight, but more effective, than TNT on a pound-for-pound basis.)

Diameter of Destruction: 100-150 m

Diameter of Damage: 300-500 m

I don't know if these numbers were estimated, or observed in tests, or observed in combat results. They are the numbers I can find.

_____

All of that said, now let's compare those metrics to Little Boy.

Explosive Equivalent: 30,000,000 lbs of TNT

Diameter of Fireball: 250-280 meters
(It was detonated in the air. If detonated at ground level, all material inside this diameter would have been vaporized.)

Diameter of Destruction: 1 Km
(Effective destruction of all re-enforced and heavily built buildings.)

Diameter of Damage: 3-5 Km
(Less damage to heavily built structures. Significant damage to lightly built structures. Roofs, windows, sometimes walls damaged. About 70-80,000 buildings were destroyed or damaged by this one bomb.)

These numbers are based on the rough surveys done of Hiroshima within about 1 week of the bombing by the Japanese, and the EXTENSIVE surveys done by the US about 6 weeks of the bombing.

_____

The explosive power of the Little Boy bomb was about 3,700 X the explosive power of a Grand Slam bomb, about 1,360 X that of the largest conventional explosive bomb ever deployed by a Western Ally nation.

To say that Little Boy in any way "had a much lesser effect" than the largest conventional bombs of WW2, or even the largest conventional bombs ever deployed for war by the British or the Americans, is either proof of someone who has no clue what he/she is talking about, or it is proof someone is trying to deceive.

You can not be off by over three orders of magnitude if you made an honest mistake. Either a fool or a lair. Those are the choices.

-Mark
(aka: Mk 1)

Mark 1

Thank you. You seem to be labouring under the impression that I'm claiming the Americans dropped a Tallboy bomb on Hiroshima. At least engage with the thread by reading through my posts and you'll see I was referring to the British research that we know was in progress, of which there were two strands: Tallboy-type concentrations of explosive power, and developments in using increasingly devastating airburst and incendiaries. Could they have developed the bomb that so much of their research was aiming at?

We know the following: Littleboy was the equivalent of 15,000 tons of TNT – a weight B-29s could carry.

We know the blast at Hiroshima was very similar to the type of air-blasts the British had been achieving using incendiary additions.

We know the British had built 20,000lbs bombs. We know that the destructive power of the largest conventional bombs is not achieved by packing in more explosive, but in the way they are constructed. The most destructive bomb ever, the air-burst MOAB bomb, for example, only contains the equivalent of 11 tons of TNT but despite its purpose being to deliver an earthquake-type shock to a limited area, it can devastate to a range of a mile.

The main devastation at Hiroshima was over a radius of 1.5 miles. Hiroshima was a mainly wooden city. There was no requirement to knock the buildings down in one blast, what was needed was a heat-blast and a firestorm.

The first question is, where was bomb research at the time leading and what size were the bombs they had?

The "Tallboy" strand simply shows how large these bombs were getting. Tallboy contained 5,200 lbs of torpex. Grand Slam contained 9,750 lbs and weighed 22,000 lbs. Modified Boeing B-29 Superfortresses could carry that weight. Not that it needed to in this case. But there was another strand of research which didn't depend on packing in explosive, but rather on methods of delivery.

So we have the R.A.F. blockbuster "Cookie" bombs, as in "cook". These used airburst or surface fuses and their primary purpose was to demolish roofs, shatter windows, and rupture gas lines so follow-up incendiary bombs could spark massive firestorms. They began with 4,000lb versions which had a destructive effect over 0.62 miles and as the war progressed, the RAF began clustering pairs or trios of bombs to create 12,000 lb city-flatteners. They were experimenting with ammonium nitrate (called Amatol), or more advanced explosive formulas like Torpex and Minol, which utilized TNT, RDX, and aluminum to boost the destructive blast.

There's no question that the trajectory of research was towards building airburst bombs with incendiary effects over greater and greater areas. So it's interesting that two of the Englishmen most intimately involved with developing conventional blast incendiary bombs, neither of whom had experience with atomic development went over to help in the Manhattan Project: William Penny, whose expertise was in TNT shock waves and the effects of high explosives combined with ammonium nitrate on cities and James Tuck — a British expert in shaped charges and conventional explosives.

Hiroshima was a highly inflammable city. So although it's interesting to know that this or that bomb didn't have the same effect of Littleboy, it's all slightly missing the point. We know that the highly secretive trajectory of bomb research was on a path to easily achieving Hiroshima-like destruction by conventional means.

"We know the following: Littleboy was the equivalent of 15,000 tons of TNT – a weight B-29s could carry"
?
Methinks someone is confusing tons with pounds.