The Boeing-built X37B Orbital Test Vehicle (OTV-2) returned from 469 days in orbit on June 16, 2012. What is was doing, no one really knows. Originally a NASA project the DoD took over the X-37 in 2004, and now it is the US Air Force’s own private secret space plane. This is the second flight of the X-37; OTV-1 spent 224 days in orbit in 2010. OTV-2 launched back on March 5, 2011.
The X-37B rides an Atlas V to orbit but returns like the Space Shuttle, gliding to a horizontal landing. The key difference, of course, is that the X-37B is unmanned. It had been previously stated that the X-37B was designed to remain in orbit for “up to 270 days”, so this latest mission indicates that either the design has been revised, or that those statements were deliberately understating the capabilities of the system. Either is plausible, given the classified nature of the program under the USAF. Boeing floated the possibility of a larger, astronaut-carrying X-37C last year, but not much is known.