Perhaps one of the rarest Harley's in existence is the model XS. Only three of this prototype military motorcycle were constructed in 1942 and of those, only one is known to remain. That single example resides in the Harley-Davidson Archives and was found in original condition with all the standard military gear still attached.
So what is an XS?
Basically, it is a sidecar rig purposely designed for use in the North African campaign during WWII. The motorcycle portion uses another prototype named the Experimental Army (XA). The XA was more or less a copy of the German BMW R71. It featured an enclosed shaft drive and side-valve flat-twin engine making it better suited for the harsh desert conditions in Africa. The XS goes one step further by adding a rear axle that uses the rear wheel to power the sidecar wheel, making a two-wheel drive system. This is the only time that Harley ever produced such a drive system. Unfortunately for Harley, neither the XA or XS ever made it to production. With the introduction of the Jeep and the end of the North African campaign, the US Army no longer needed either model.