As @reirab explained, "N" registration numbers are generally used for civilian aircraft. However, there are some notable exceptions.
The de Havilland DHC-7 (aka "Dash-7", variously nomenclatured O-5A, EO-5B/C, or RC-7B) is used by the United States Army as its Airborne Reconnassaince-Low manned aerial imagery intelligence platform.
Image credit: Alan Radecki / Wikimedia Foundation
Another de Havilland, this one a DHC-6, is used by the Air Force Academy's parachuting team:
Image credit: Ian Tate / Flickr
Dover AFB's 436 Force Support Squadron employs several Cessnas at their Aero Club, used for flight training. One of them made the news when it made an emergency landing onto a local highway.
Image credit: Adrian R. Rowan / Dover AFB
These are just a start. Getting creative with search queries, you can also find hundreds of other aircraft listed in the FAA registration database.
Virtually all of these are very special-purpose aircraft, with no munitions or other armament (aside from chaff/flare as in the case of the Dash 7).
And although not technically military (yet), it is common for developmental aircraft to be given civilian registration numbers during flight testing phases prior to final delivery. This prototype KC-46A, painted mostly in military-style "livery", has registration number N461FT.
Image credit: Ken Fielding / Wikimedia