In the 1930s BMW were producing a number of popular and highly effective motorcycles. In 1938 development of the R75 started in response to a request from the German Army. Preproduction models of the R75 were powered by a 750 cc side valve engine, which was based on the R71 engine. However it was quickly found necessary to design an all new OHV 750 cc engine for the R75 unit. This OHV engine later proved to be the basis for subsequent post-war twin BMW engines like the R51/3, R67 and R68.