Albert Friedrich was granted permission to develop the Unimog in November 1945, and entered a production agreement with Erhard und Söhne (Erhard and Sons) in Schwäbisch Gmünd on 1 December 1945. Development began on 1 January 1946. Soon after, Heinrich Rößler, the Unimog lead designer, joined the development team. The first prototype was ready by the end of 1946. The early prototypes were equipped with the M 136 Otto engine, because the development of the OM 636 Diesel engine had not been finished. The prototypes were similar to the later series production models. The original track width of was equivalent to two potato rows.