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 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 60 centimetres (24 in), which required adding a sixth roadwheel, and widened by 32 centimetres (13 in) to better accommodate the gun while preserving its low silhouette. 15 millimetres (0.59 in) plates formed the front and sides of the open-topped fighting compartment, which was also open at the rear. Its sides were notably lower than the front which made the crew vulnerable to small arms fire and shell fragments.