The Russian IS-1 Heavy Tank lineage starts with the KV-13, which was another attempt to merge the high mobility of a medium tank and the armoured protection of a heavy tank, much like the KV-1S. The KV-13 had failed its trials in 1942, however in December 1942 the assembly of two new prototypes of the KV-13 began at the Experimental Tank Factory.

They shared only the hull, torsion bar suspension, and chassis from the first KV-13 prototype. The turrets and many other elements were completely new designs.

1 Prototype was fitted with a 76.2mm main gun, however it was soon realised that a better gun would be need to deal with the German Tiger, so the vehicle was fitted with the 85mm D-5T, which was the Tank version of the 52-K 85mm Anti-Aircraft gun.

This prototype (factory designation: Object No.233) had better armour protection, a higher speed and  better armament than the KV-1S.

The Object No.233 was accepted into service by the Russian Army and designated the IS-1. 107 IS-1’s were built between October 1943 to January 1944 when production ended in favour of the IS-2.