The KV-1 was the first of a series of Soviet Heavy tanks named after Marshal Kliment Voroshilov. At the time of its introduction in 1940 it the only true heavy tank design aside from the French Char B1 and it was by far the most heavily armored tank in the world, being largely impenetrable to most German ammunition (both towed anti-tank guns as well as tank guns) at the time of Operation Barbarossa. At the same time, it's 76.2mm gun was easily capable of destroying the German medium tanks it faced. Suspension was provided through independent torsion bars on the six road wheels, and wide tracks gave it low ground pressure for a tank of its weight (it was roughly twice as heavy as a German medium). Although speed was adequate for a medium tank, there were some significant issues with the clutch and transmission, and it was unable to change gears while moving. Crew visibility was also poor when the hatches were closed, and the commander was also in charge of loading the gun. Prototypes of the KV-1 were in action during the Russo-Finnish war where its performance was good enough to be selected for production over rival designs. A few hundred were in service by the time of the German invasion where, along with the T-34, they came as a shock to the Wehrmacht and prompted the development of more powerful weaponry, such as the Panther, to defeat them. The KV-1 remained in service throughout the war and in production until 1943 by which time a new generation of heavy tanks, the IS series, was being developed.

The prototype of the KV-1 competed against two other rival designs, the SMK and the T-100, and trials were undertaken directly during the Russo-Finish War. There it was decided that it was superior to the others and ordered into production. The initial Model 1939 was armed with short L-11 gun due to delays in delivering the higher velocity F-32 gun. This was finally fitted on the Model 1940 which also featured a more powerful engine as well as extra bolted armor plates on the turret and hull. An even more powerful ZiS-5 gun was used on the Model 1941 while the Model 1942 had a cast turret and thicker armor that reached a maximum of 120mm. The increase in armor protection resulted in a loss of speed, and this was rectified in the KV-1S which had thinner armor as well as other mechanical improvements, this was the last version of the KV-1 to be produced. A flamethrower version was known as the KV-8.