The heavy armour of the latest Soviet tank designs ensured that they made a major impact on the battlefield, even in the hands of the hastily-trained conscripts that were all that was available after the disasters of 1941. When the Germans started fielding tanks armed with new long-barrelled 7.5cm  guns, there was an urgent need for a better armoured and easier to use tank than the T-34 medium tank.

In June 1942, the GABTU (Main Directorate of Armoured Forces) ordered work to begin on two prototypes. The SKB 2 heavy tank design bureau in Chelyabinsk started the KV-13 program, which eventually became the IS-1 heavy tank. The Morozov Design Bureau that had been responsible for the T-34, now working at the Uralvagonzavod complex, developed the T-43 based on the advanced T-34M project, which had been cancelled in 1941 when Germany invaded the USSR. The first prototype T-43 was finally completed in March 1943. Thicker armour means a heavier tank, so the designers made the tank as small as possible to minimise the amount of armour needed.