Russian KV-3 tanks feature 120mm frontal armor and 90mm of side protection. This comprehensive protection was even more powerful than the German Tiger tanks put out a year later. Design requirements improved the artillery power to a Navy B-34 type 107mm gun, later to become the ZiS-6-type gun. Series production was intended to start in late 1941, but the German invasion of the USSR halted these plans and the only prototype hull was destroyed. The design was accepted for service in May 1941 and was to have entered production at the Kirovsky Plant in August but the German invasion forced this to be abandoned.