The earliest KV-1 were produced in 1939. At the time of the German invasion of the Soviet Union, 500 KV were in service. The Model 1941, KV-1B had extra armour added to the turret, hull front and sides. The turret was cast instead of welded and the KV-1B was armed with the long barrel F-34 and later the 76.2mm ZIS-5 gun.