Sometimes referred to as the Sturmpanzer II Bison, the 15 cm sIG 33 auf Fahrgestell Panzerkampfwagen II (Sf) was a German self-propelled heavy infantry gun used during World War II. The 15 cm sIG 33 (Sf) auf Panzerkampfwagen I Ausf B built in time for the Invasion of France in 1940 had proven to be too heavy for their chassis as well as enormously tall. The same gun was mated to the Panzerkampfwagen II chassis in an attempt to drastically lower its height while using a stronger chassis. The prototype used a standard Panzer II Ausf. B chassis when it was built in February 1941, but this was too cramped for use. The chassis was lengthened by , which required adding a sixth roadwheel, and widened by to better accommodate the gun while preserving its low silhouette. plates formed the front and