During the combats in the summer of 1943, the Soviets were aware of the shortcomings of their heavy KV tanks. Then, began the program for a new tank under designation “Izdeliye 237” (Object 237), for which some modified components of the canceled KV-13 and KV-1S tanks would be used. In addition, a new turret with reinforced armor and new optics was built based on that of the KV-1S tank. The new project was baptized as “IS”, in honor of Iosif Stalin and would be carried out by the “IS Bureau” under Nikolai Shashmurin command. Finally, in August 1943, the “Object 237” prototype tank, (armed with the 85mm D-5T tank gun), was presented and tested at Kubinka proving ground before Stalin. Following the presentation Stalin ordered production to begin as soon as possible under the name “IS-85 heavy tank”. It was necessary to modify the vehicle’s hull and suspension, which produced delays in the first deliveries. At the same time, it was decided to arm the new IS-1, (as it was finally baptized), with the new 100mm D-10 gun instead of the 85mm D-5T. In November 1943, the first IS-1 tanks left the production line with both the D-10 or D-5T guns installed. At the beginning of 1944, 107 tanks had been delivered, but it was decided to convert 102 IS-1 tanks to the new up-gunned, (with the 122mm D25-T gun), IS-2 model. In this way, almost none of the IS-1 tanks were delivered to combat units, so their life was really fleeting.
IS-1 / IS-85 gallery and more info