Military planners directed the design bureaus of both Gen. Vasiliy Grabin and Gen. Fyodor Petrov to modify the 85 mm anti-aircraft gun for use as an anti-tank weapon. Petrov's bureau developed the D-5 85 mm gun. Though much too large for the T-34 or KV-1 turret, it was thought the gun could be mounted upon the chassis of the SU-122 self-propelled gun to give the weapon mobility.