The IS-1 is the first variant of the IS heavy tank family. The new IS-1 was intended to overcome the lack of firepower and protection issues of the KV series with a whole new sloped hull design. This was a strong armour design intended to deflect or withstand projectiles from all sides. The only major design requirement was that the weight not surpass that of the interim KV-85. The new armament was the same as the KV-85, an outstanding 85 mm D-5T tank gun with significantly higher range and muzzle velocity than prior Soviet heavy tanks.