The Sturmpanzer (Bison) series were Germany's first self-propelled guns to be deployed in World War II. They were built around the 15cm sIG 33 heavy infantry gun, whose original towed version proved difficult to operate effectively with fast-moving Panzer formations. The Sturmpanzer I used the chassis of a Panzer I Ausf B with the superstructure and turret removed. The gun mount was protected by particularly large gun shield that gave it its distinctive shape, and offered protection from the front and sides but was open on the top and at the rear which made the loaders vulnerable to enemy fire (no defensive armament was provided either). The unconventional shape of the gun shield led to frequent breakdowns and it also suffered from an inability to carry ammunition, requiring a separate ammunition carrier. As a result, the Sturmpanzer II was built using the larger Panzer II chassis, which was widened and lengthened (with a sixth road wheel added). It featured a much lower superstructure although it retained both the open-top and open-rear. However, even the sides proved too low to provide adequate protection to the crew although on the positive side, it was able to carry its own ammunition. The Sturmpanzer I entered service shortly before the Blitzkrieg campaigns against France and the Low Countries, with a total of six Panzer divisions. Although highly effective during that campaign, most were subsequently lost during the first year of the war against Russia. The Sturmpanzer II saw service in North Africa with the 90th Light Division until the Tunisia surrender.

The 15cm sIG33 (Sf) auf Panzerkampfwagen I Ausf B (known unofficially as the Sturmpanzer I or Bison) was converted from Panzer I Ausf B chassis in February 1940 making them ready for action by the time the campaign against the Western Allies was resumed in May. Production of the 15cm sIG 33 auf Fahrgestell Panzerkampfwagen II (Sf) (unofficially known as the Sturmpanzer II) took place in the final two months of 1941. It is often described as an assault gun, although its limited protection would not make it fully suitable for that role. No variants of either vehicle were produced.