The best military bike of World War II was almost certainly Germany's BMW R75. This machine had oil-damped telescopic front forks, rather than the primitive girder suspension of the day. And with shaft drive and superior cooling the flat twin performed exceptionally, especially in the African desert.
The Japanese were obsessed with their knock-off Harleys, while the Italians used a rugged 500cc forward-facing flat single - the Moto Guzzi Alce. So, mysteriously (to me anyway) the Axis powers did not seem keen to share the R75 concept, but their enemies knew a great design when they saw it.
The Russian M72 was a near clone of the R75 and the US Army asked both Harley-Davidson and Indian to see if they could produce a similar shaft-drive motorcycle. In 1942, Harley replicated the BMW engine and transmission, converted metric measurements to inches and produced its version - the 750cc shaft-drive XA.
These BMW R75 knock-offs apparently performed quite well, but Indian developed a better-looking shaft-driven design - a transverse V-twin reminiscent of a post-war Moto Guzzi.