With experience gained during the initial phases of the Battle of Stalingrad, in September 1942 the Wehrmacht's arms bureau, the "Waffenamt", called for a new standard for heavy assault guns: 100 mm of armor to the front, 40–50 mm on the sides, wider tracks, ground clearance of 50 cm, top speed of 26 km/h and the lowest possible firing positions. The new "Panzerjäger" ("tank hunter") design would be armed with the same 7.5 cm gun as fitted to the Panther: the Pak 42 L/70. Initially a new chassis was planned, but that of the Panzer IV had to be used.