This is the RQ-180, the Pentagon's new super-spy plane, which has been secretly developed for years. Unlike Lockheed Martin's SR-72 Son of Blackbird, Aviation Week & Space Technology reports that this stealth beast is already flying across the restricted airspace over Area 51. This seems to be its home at the not-so-secret base:
Aviation Weekly's Senior Pentagon Editor Amy Butler and Senior International Defense Editor Bill Sweetman got the skinny, along with details about its location. The plane, now in the flight testing phase, will participate in real world missions as soon as 2015. Apparently, the RQ-180 has been in development many years without anybody knowing about it—perhaps as early as 2006.
Defense Tech notes that the hangars can hold "an aircraft with a wing span of more than 130 feet at Northrop's Palmdale, California, site and at the Air Force's Area 51 test center in Groom Lake, Nevada," which makes it "much bigger than Lockheed Martin Corp.'s RQ-170 Sentinel, which has an estimated wingspan of between 65 feet and 90 feet," the plane used in the operation that killed Osama Bin Laden.