In April 1941, it was decided to create a project for a new Super-heavy tank. It was supposed to be able to defeat any German heavy tank. The project was intended to remain an unbuilt design, until it was needed. Several tank designers were told to create basic concept drawings. Sources differ as to the designers involved.
Basic armour was to be 125 to 130 mm and up to 150 mm over vulnerable areas. A new gun was required. There were about 20 different designs generated by the project. The lightest design was 82.5 tonnes and the heaviest was 107.7 tonnes. All had a 107mm ZiS-6 main gun and all but one had a second tank gun. The second gun was a 45 mm 20-K, or a 76 mm gun. Solutions to the positioning of the guns – turrets or hull mountings – varied. Crew requirements were from 5 to 9 men.
The 107 mm ZiS-6 guns
The F-42 107 mm tank gun was created in Plant #92, in a design bureau under the supervision of V. Grabin. The F-42 was based on the F-39 95 mm tank gun (see 107mm divisional gun) and it was finished in the autumn of 1940. By the beginning of 1941, the new gun had been factory tested in a field casemate. In March 1941 it was tested in the turret of a KV-2 tank. The ZiS-6 gun passed factory testing by June 1941, after which it was sent to the Artillery Scientific Test Range at Leningrad.
According to V. Grabin ("Oruzhie Pobedy" - "Weapons of Victory") about 600 ZiS-6 guns had been finished by the autumn of 1941. Those guns were all destroyed because the tanks were never going to be made. The plants chief designer wrote that "five serially-produced ZiS-6 cannons were manufactured in July–August 1941, after which their production was halted because the heavy tanks were not ready". Those documents had been signed by V. Grabin.