After encountering early German Tiger I prototypes at Leningrad in 1942, the Soviets realized the F-34 tank gun was becoming inadequate. The development of heavier cannons, abandoned at the outbreak of the war, was resumed; among them was a program to convert the 85 mm AA cannon into a high-velocity anti-tank gun - the ZiS-S-53. Although initially it was considered impossible to create a T-34 turret that could house an 85 mm gun - leading to the turretless SU-85 TD - eventually such a design was implemented, adding a separate gunner as well to considerably improve situational awareness, which was hampered as the commander was constantly occupied with manning the main gun.