15 cm sIG 33 (Sf) auf Panzerkampfwagen I Ausf B

The 15 cm sIG 33 (Sf) auf Panzerkampfwagen I Ausf B (sometimes referred to as the Sturmpanzer I Bison ) was a German self-propelled heavy infantry gun used during World War II.

Development and history
The Invasion of Poland had shown that the towed sIG 33 guns assigned to the infantry gun companies of the motorized infantry regiments had difficulties keeping up with the tanks during combat. The easiest solution was to modify a spare tank chassis to carry it into battle. A sIG 33 was mounted on the chassis of the Panzer I Ausf. B, complete with carriage and wheels, in place of the turret and superstructure. Plates 13 mm thick were used to form a tall, open-topped fighting compartment on the forward part of the hull. This protected little more than the gun and the gunner himself from small arms fire and shell fragments, the loaders were completely exposed. The rearmost section of armor was hinged to ease reloading.

There was no room to stow any ammunition so it had to be carried by a separate vehicle. When mounted, the sIG 33 had a total 25° of traverse and could elevate from -4° to +75°. It used a Rblf36 sight. The chassis was overloaded and breakdowns were frequent. The vehicle's extreme height and lack of on-board ammunition were severe tactical drawbacks.

Thirty-eight were produced in February 1940 by Alkett. Thirty-six of these were organized into independent schwere Infanteriegeschütz-Kompanie ("Self-propelled Heavy Infantry Gun Companies"); mot.S. Numbers 701-706 and these were assigned to the 1st, 2nd, 5th, 7th, 9th, and 10th Panzer Divisions in the Battle of France as well as Operation Barbarossa, (the invasion of the Soviet Union). The 705th and 706th were destroyed at this time, belonging to the 7th and 10th Panzer Divisions respectively. Of the remaining companies, only the 701st participated in the opening stages of the subsequent Case Blue in 1942, although it, and its parent 9th Panzer Division, were transferred to Army Group Center by the end of the summer of 1942. The last reference to them is with the 704th Company of the 5th Panzer Division during the middle of 1943.