The idea for a heavy infantry support vehicle capable of demolishing heavily defended buildings or fortified areas with a single shot came out of the experiences of the heavy urban fighting in the Battle of Stalingrad in 1942. At the time, the Wehrmacht had only the "Sturm-Infanteriegeschütz 33B" available for destroying buildings, a "Sturmgeschütz III" variant armed with a 15 cm sIG 33 heavy infantry gun. Twelve of them were lost in the fighting at Stalingrad. Its successor, the "Sturmpanzer IV", also known by Allies as "Brummbär", was in production from early 1943. This was essentially an improved version of the earlier design, mounting the same gun on the Panzer IV chassis with greatly improved armour protection.