The US Air force has put three different X-37 space planes into orbit. We still aren't sure exactly what the spacecraft are doing in orbit. The last of the X-37B spacecraft to launch was the OTV-3 or Orbital Test Vehicle 3 mission. The spacecraft launched on December 11, 2012.
That means that OTV-3 is only days away from having spent a full year in orbit. The payload that the spacecraft took into orbit and the mission of the spacecraft is classified. This particular X-37B spacecraft was used in 2010 for the spacecraft's first mission called OTV-1.
The spacecraft was in orbit for 225 days on its first mission. The next X-37B mission, OTV-2, used a different spacecraft. That mission was conducted in 2011 and spent 469 days in orbit. All of the missions so far have glided back to Earth and landed at Vandenberg AFB in California.
So far, OTV-3 has orbited at two different altitudes. The spacecraft changed orbit altitude about 11 weeks after it was put into orbit. X-37B spacecraft look similar to the retired space shuttle fleet, but are significantly smaller. The X-37B is 29-feet long and nearly 15 feet wide.