IS stood for Iosef Vissarionovich Stalin / Iosef Stalin.
The need to increase the firepower of the KV series was seen by the end of 1942 with the Germans introducing Tiger tanks a new tank was needed to combat it. The IS-85 was intended to be a 4-man tank, with an 85 mm gun, not weigh more than a KV-1, and that had thick enough armor to defeat the German 50 mm anti-tank gun.
The IS-85 was designed by a design bureau led by Lieutenant-Colonel Kotin. He split the bureau into two and each team had different specifications to design for. One of the teams was told to improve the KV series as a stopgap for a new heavy tank. This design was to become the KV-85. The other team was to design the new heavy tank, the IS-85. The team was headed by, released from a labor camp, N. V. Tseits.The design was initially called the KV-13 and had a redesigned hull and chassis from the KV series.
Based on the KV series it had a new hull, improvements in the suspension, transmission, and power train. The turret was cast and the hull had cast and rolled parts welded together.
The IS-85's design first started out with same 85 mm gun as the T-34/85 and a few were produced in late 1943. The cast turret (which was used in the KV-85) was designed by General A. A. Petrov. A heavier gun was needed as the T-34/85 was being produced and having a heavy tank with the same armament as a medium was thought to be inefficient. The 100 mm designed by V. Grabin was tried with great success and known as the IS-100. The 122 mm was tried but it was only a little more effective, however the 100 mm was in short supply. The superstructure extended out over the tracks and this allowed for a bigger turret.
There was no machine gun in the hull which allowed for a better ballistic shape.