From Foreign Policy:

The RQ-180 is likely flying from the secret Air Force test facility at Groom Lake, Nevada, widely known as Area 51. Its exact specifications, including such crucial details as the number of engines, is unknown, but Aviation Week suggests a wingspan of over 130 feet, based on hangar construction at Northrop’s Palmdale, California facility. The number of aircraft built is also unknown; however, a flight test program, relatively quick entry into service and open budget documents suggest a small fleet are flying routinely.