This video, created by the US Signal Corps. during WWII, touts the 1942 production of the Harley Davidson XA. You may notice some similarities to the BMW R71, first released in 1938, from which the XA was developed (according to an interview with William Davidson Jr. in the early 1990s).
"Because the fact that the engine is of the opposed piston type makes for better balance and smoother operation."
"Heretofore, the problem with developing a motorcycle driveshaft that could successfully stand the rough treatment the machines must take had baffled engineers."