The KV tank was a new type of heavy tank, a rejection of the multi-turreted designs like the T-35 and T-28 that had dominated the 1930s. During combat tests against the Finns during the Winter War, the new design proved itself superior to multi-turreted SMK and T-100 designs.
In 1939, a production run of 50 was ordered. When Operation Barbarossa began, the Red Army had 508 new KV tanks. So effective was its armor, that the Germans were incapable of destroying it with their tanks or anti-tank weapons and had to rely on pioneers with anti-tank mines, 8.8cm FlaK36 anti-aircraft guns, or s10cm K18 guns to knock them out. German advances were brought to a halt while pioneers or guns were moved up to deal with the seemingly immovable obstacles. On June 23, 1941, a single KV-2 tank of 3rd Mechanised Corps held up the entire 6th Panzer Division for one day during the Battle of Raseiniai.