Germany was a leader in self propelled artillery at the start of WWII. Approaching the idea of motorized artillery from a new angle, they ended up with a wide spectrum of SPGs by the middle of the war. This also applied to self propelled howitzers, which were first used in France in 1940. Truly mass produced SPHes entered service closer to mid-1943. These were the Grille (Grasshopper), the best SPG series with the 149 mm sIG 33 howitzer.
Best chassis for a big gun
The idea to install the heavy 149 mm sIG 33 howitzer on a self propelled chassis was implemented in early 1940. Alkett designed the 15 cm sIG 33 (mot S) auf Pz.Kpfw.I Ausf.B on the chassis of the Pz.Kpfw.I Ausf.B. The conversion was fast to develop, but not particularly good. The resulting vehicle was too tall and too heavy, so the cannon was often unloaded and towed behind the vehicle during travel.
Understanding that the Pz.Kpfw.I chassis is ill-suited for such a big and heavy gun, Alkett began working on an analogous vehicle using the Pz.Kpfw.II Ausf.C chassis. This time the wheels were removed, which reduced the height of the SPG. However, trials showed that it was too cramped inside and that the Pz.Kpfw.II chassis could not be used in its initial form. This led to the 15 cm sIG 33 B Sfl, a lengthened Pz.Kpfw.II chassis with a new engine. This resulted in a roomier fighting compartment. However, the 15 cm sIG 33 B Sfl was not particularly successful. The vehicle carried only 10 rounds of ammunition. The mass of 16 tons and a power to weight ratio of under 10 hp/ton resulted in not only low performance, but low reliability. It is not surprising that the 15 cm sIG 33 B Sfl was produced in even smaller numbers than its predecessor. Only 20 units were built.