Back in the early 1940s, the US Army asked Harley-Davidson to design a next-generation military motorcycle although the company was already producing the WLA, based on its traditional 45-degree V-twin engine. The army ordered a test batch of 1,000 XAs from Harley-Davidson. At the same time, the military also asked Indian to make a 750cc shaft-drive twin, and it came up with a 90-degree V-twin design.
The idea was to put both new machines through their paces, and award a lucrative military contract to the winner.

Eventually, the army finished its testing, and decided that neither new bike would be built. Instead, they bought several thousand more Harley WLAs. In the end, the military decided to hitch its hopes to a vehicle that could go through anything, didn’t tip over, and required very little training to operate — the Jeep.

Blame the Jeep. That’s why military motorcycles of any kind fell out of favour during World War II.
GI Joe US Army WLA 45 Harley-Davidson Military Motorcycle by HASBRO 1998