The Harley-Davidson XA (Experimental Army) was a flat-twin shaft drive motorcycle made by Harley-Davidson for the US Army during World War II.[1]
During WWII, the Army produced a specification for a motorcycle much like the BMWs used by German forces, with shaft drive, a boxer engine, and several other features that made the BMWs exceptionally reliable and low-maintenance machines. Harley-Davidson produced the XA, which was based closely on the BMW. Though an excellent machine, only about 1000 were produced. The XA was expensive and, by the time production had begun, it was clear that the Jeep was the Army's general purpose vehicle of choice. The less advanced but cheaper WLA was considered sufficient for its limited roles.
According to the American Motorcyclist Association's Motorcycle Hall of Fame Museum, regarding the World-War-II-vintage Harley-Davidson XA, which has an opposed twin engine — "Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature 100 °F (56 °C) cooler than a standard Harley 45."[1] The latter was a transversely mounted V-twin, in which the air stream cannot reach the cylinders, especially the rear cylinder, as efficiently as on longitudinally mounted flat twin.