Towards the end of September 1939, Krupp designed the "first real self-propelled artillery piece", the Sonderkraftfahrzeug 165/1 (Special Purpose Vehicle 165/1, abbreviated Sd.Kfz. 165/1). After a series of tests, the Sd.Kfz. 165/1 was accepted by the Wehrmacht in early January 1940. In 1941, Krupp built prototype vehicles armed with the 105 mm leichte Feldhaubitze 18/1 L/28 (light field howitzer 18/1 L/28, abbreviated leFH 18/1 L/28) cannon based on a modified Panzer IV chassis. The prototypes were fitted with a smaller six-cylinder Maybach HL66P engine, which had a power capacity of 188 hp (140 kW). Although 200 vehicles were ordered, Krupp completed only 10 prototypes in the final four months of 1942. These saw service on the Eastern Front.