"The idea to use tractors as a chassis for SPG was born in the USSR in the 1930s. The SU-2 and SU-4 prototypes were built, but the projects did not move past the prototype stage. On the other hand, the Germans achieved different results. Taking captured French Renault UE tractors, they created SPGs with 3.7 cm Pak guns. The resulting vehicle wasn't the best, but could be produced in large amounts at a small cost. A year later, the ZIS-30 was built in the USSR, the first mass production SPG of the war.
Anti-tank Ersatz
The USSR first began seriously considering using artillery tractors as a chassis for tank destroyers in the spring of 1941. The first tractor considered for the role was the STZ-5. A more powerful ZIS-16 engine was proposed to increase mobility, as well as a longer chassis to add stability. The 57 mm ZIS-2 anti-tank gun, currently in the pre-production stage at factory #92, was proposed as the armament.
The Voroshilovets heavy artillery tractor was also considered as a chassis for a tank destroyer. An 85 mm mod. 1939 (52-K) AA gun was to be installed in its cargo compartment. Both vehicles would be partially armoured…"
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