Early in World War II, Soviet tanks such as the T-34 and KV-1 had enough firepower to destroy any German tank then available. However, in the fall of 1942, Soviet forces encountered the new German Tiger tank, with armor too thick to be penetrated by the 76.2 mm guns used in the T-34 and KV tanks at a safe range. By spring 1943, the Soviets had also received reports of the new Panther tank, although the Panther was not seen in combat until July 1943, during the battle of Kursk. This new generation of German vehicles meant the Red Army needed a new and more powerful tank destroyer.
In May 1943, work was begun on both a new anti-tank gun and a redesign to the armament of the SU-122. Developers of the former put their efforts to adapting the 85 mm heavy anti-aircraft gun, one of two types identified as best against the Tiger (the other being the 122 mm A-19). The SU-122, meanwhile, was rearmed with an existing 85 mm gun, the S-18, which was itself improved in the process. The production factory at Uralmash, which received the SU-122 design, attempted to reject the design as too expensive, since the larger gun breech meant that the entire hull would need to be modified, but it was required nevertheless to put the design into production.
Of the SU-85, several prototypes were rejected for design flaws, but after several changes, which included changing the gun to a D-5T, one was put into service as the SU-85.[1] At the same time, the 85 mm D-5S, which had proven both effective and reliable, was modified to include a telescopic sight and a new ball mantlet design and retitled the SU-85-II.
