In 1943, the Kirov factory in Chelyabinsk (ChKZ) developed a heavy tank armed with the 122 mm D-25T gun. This tank was indexed IS-2 (or IS-122, by the gun's caliber). The tank was not very well balanced when it came to armament and protection. The gun was indeed powerful, but its armour was not impervious to German anti-tank guns. This issue of protection was more or less resolved with a straightened front plate. Protecting the turret in this way was difficult due to design limitations.
In December of 1944, the Kubinka proving grounds saw the first prototype of the "Kirovets-1" tank designed in Chelyabinsk, which had improved armour compared to the IS-2 tank. The tank passed trials in a satisfactory manner, but the commission remarked that the tank was equivalent in many ways to the currently produced IS-122. Additionally, the engineers were tasked with removing a number of defects (for example, reliability of the ventilation system).
In the fall of 1944, at factory #100, a group of engineers led by G. Moskvin produced their own modernized heavy tank. As with the Kirovets, they put a lot of thought into the protection of their vehicle.
The People's Commissariat of Tank Production suggested that the best parts of both designs be used, On December 16th, 1944, an order was issued, describing further work on the "complete and improved" heavy tank.
On February 20th, 1945, work on the "Object 703" completed, and it was sent to Kubinka for trials. The full set of technical documentation for this tank, already indexed IS-3, was signed on May 21st of that year. The tank was too late for the Great Patriotic War.
A little over 2300 IS-3 tanks were built. Due to its characteristic two-piece upper glacis plate, it was nicknamed "pike". Tanks of this type were sent to heavy tank and SPG regiments of the Soviet Army. 52 IS-3 tanks from the 2nd Guards Tank Army took part in the parade through Brandenburg Gate on September 7th, 1945.
Original article available here.