In 1942 the Soviet intelligence had information of the new German tanks Panther V and Tiger VI, and this resulted in a need of a new Russian tank to even the score in fire power and protection. At first it was planned to up-armour the T34 m/43, but instead a larger turret was put to the same chassis that was altered inside.
Two prototypes were built, known as T43 and they had a commander's cupola on top. At the beginning of 1943 they were tested and turned out to be just a little better in combat than T34. The decision was taken to start production, but after the battle of Kursk a few months later the it was obvious that a brand new heavy tank model was needed instead (it was to be IS-1) and thus the T43 project came to a close.