The Red Army had possessed dedicated anti-fortification vehicles in the pre-war period, such as the KV-2 heavy tank armed with the 152.4 mm M-10 howitzer. Mass production of KV-2s ceased in October 1941, when the Kirov Works had to be evacuated from Leningrad to Chelyabinsk. Some made it to 1942 but their actual number remains unknown. When the necessity for a new heavy breakthrough vehicle became apparent in the Soviet offensives, a new anti-fortification vehicle was designed with the same purpose in mind, but with higher mobility, heavier armor, reduced production cost, and the more powerful and accurate ML-20 152mm gun. Mounting the ML-20 in a turret was impossible due to its length and recoil, and it was eventually decided that the new vehicle should have a non-rotating gun mounted in a fixed casemate-style superstructure.