Jorge R. Frank wrote:
Consider the possibility that the "X" in the vehicle name really is
accurate: it's an experimental vehicle, not directly a prototype for a
particular operational vehicle (which would have a "Y" prefix).

A lot of the other military X-planes that have been recently made are
prototypes for operational weapons systems.
What makes X-37B interesting is that originally NASA was supposed to
have some use for this spacecraft also in the X-37A version.
Here's some more info on it:
http://www.designation-systems.net/dusrm/app4/x-37.html

Pat