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The KV-3 was a more heavily armoured version of the KV-1 that accepted for service in May 1941 but cancelled after the German invasion of the Soviet Union.
The KV-3 was developed in response to German propaganda films showing French Char B1 bis heavy tanks had been penetrated by shells. As a result it was decided to develop a more heavily armoured version of the KV-1, initially as Obiekt 222. The chance was also taken to make a few general improvements.
The new vehicle had thicker armour, with the hull and turret now getting 90mm of armour. This raised the weight to 51 tonnes, so it was given a 700hp V-5 engine instead of the 600hp engine used in the KV-1.
The turret was modified. It was given a commander’s observation cupola, and was to be armed with the ZIS-5 76.2mm gun. The third crewman in the turret moved from the rear, where he had been the assistant driver and rear machine gunner, to become the loader, allowing the commander to concentrate on his job.
One prototype was built, armed with the F-32 76.2mm gun used on the KV-1 Model 1940. This prototype underwent trials, and was then accepted for service in May 1941. Production was to start at the Kirovskiy Plant in Leningrad in August 1941. However after the German invasion of the Soviet Union the KV-3 was cancelled to avoid disrupting production.
Some of the features introduced on the KV-3 were introduced on later versions of the KV-1, including the heavier armour and the ZiS-5 gun. However the KV-1 never got the improved turret or the more powerful engine,
A second design, Obiekt 220, is sometimes also designated as the KV-3. This was a more extensively modified vehicle, with a longer hull, more powerful engine, and heavier gun. Initially it was to be armed with a 85mm tank gun, but Marshal I Kulik, head of the GAU, incorrectly believed that the Germans were about to massively increase the armour on their tanks. In March 1941 he insisted that the Obiekt 220 be armed with a new ZiS 6 107mm gun, which was still under development. At least one prototype of the hull was completed in the early summer of 1941, but the new gun wasn’t ready, so they were given F-30 85mm guns instead. One of these saw combat in Leningrad in 1941. The 107mm gun was tested in a KV-2 instead, but never installed in a production tank. Obiekt 220 was rejected for service because of its weight and problems with its drive train, and it is unclear if it was ever known as the KV-3.