In the summer of 1942, S.J. Kotin and other designers, were given the task to develop a medium self-propelled gun initially called the SU-35.
The Central Artillery Directorate (GAU) in April 1942 gave instructions to several design teams to work on a self propelled vehicle that could carry the 122 mm gun.
The five man crew were able to work inside the roomy (compaired to tanks with turrets) superstructure.
The gun was held in a cast mantlet with fabricated armor cover for the recoil system. Excelled at attacking strong points but was weak on antitank performance. HEAT ammunition was developed but is was only effective at short ranges and still wasn't capable of piercing much armor.
Late models had similar ball mantlet as the SU-85.