The SU-122 was a Soviet self-propelled howitzer or assault gun used during World War II. The number "122" in the designation represents the caliber of the main armament-a 122 mm M-30S howitzer and the chassis of the T-34.
The Soviet High Command became interested in assault guns following the success of German Sturmgeschutz IIIs. Assault guns had some advantages over tanks with turrets and the lack of a turret made them cheaper to produce. They could be built with a larger fighting compartment and could be fitted with bigger and more powerful weapons on a given chassis.
A prototype assault gun, armed with the 122 mm howitzer and built on the German Sturmgeschütz III chassis was developed, designated SG-122. Production was halted when the vehicle was found to be hard to maintain and judged to be unsuccessful. Simultaneously, an SPG based on the T-34 medium tank was also developed. Initially, the T-34's chassis was selected for the 76.2 mm F-34 gun. This vehicle, the U-34, was created in the summer of 1942 at UZTM design bureau. It was a tank destroyer with the same armament as the T-34, but without a turret. The vehicle was 70 cm lower than a T-34, had thicker armor, and was 2 tonnes lighter. The five-man crew consisted of a driver, gunner, commander and two loaders.
By 25 November 1942, the first U-35 prototype was ready. Trials ran uncovered various faults in the design, including insufficient elevation, a flawed shell transfer mechanism, poor ventilation for the crew compartment, and the fact that the commander had to assist in operating the gun. The U-35 entered service with the Red Army as the SU-35 (later renamed SU-122) despite these faults. Production SU-122s were based on an improved prototype built after trials were conducted. They incorporated several modifications, including slightly less sloped front armor to ease production, modified layout of the fighting compartment, fewer vision slots, and a periscope for the commander. The first production vehicles were completed before 1943.