The Russian KV-13 tank 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, however the KV-13’s armour was constructed with cast steel (the turret and most of the hull) which increased the effective armour protection with the added bonus of reducing the amount of steel need to build the vehicle.
The first prototype was delivered for trials in May 1942. It was equipped with the 76.2mm ZiS-5 main gun, 120mm thick hull armour and a 600hp V-2K engine, which gave the vehicle a reported top road speed of 55km/h.
The trials highlighted issues with the transmission resulting in poor power to weight ratio, tracks and rollers also broke. A new transmission was sought as well as other improvements and the vehicle was trialled again with out much success, only to be rejected by the army. The KV-13 did not enter production.
Despite this, 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 prototype mentioned above. The turrets and many other elements were completely new designs. These two prototypes went on to form the IS series of heavy tanks, aka Josef Stalin tanks, aka JS tanks, the IS-1 and IS-2.