The Air Force is believed to have only two X-37B space planes. These space planes have flown at otal of three missions, which are known as OTV-1, OTV-2 and OTV-3. (“OTV” is short for Orbital Test Vehicle.) The first mission blasted off in April 2010, and the craft circled Earth for 225 days. The second X-37B vehicle launched in March 2011, performing the OTV-2 mission. This spaceflight lasted 469 days, ultimately landing at Vandenberg Air Force Base in California in June 2012. That was the same landing site OTV-1 used after completing its mission.
The current OTV-3 mission is reusing the first X-37B space plane from the OTV-1 flight, showcasing the reusability aspect of the program.
schwit1 writes: The Energizer bunny of space: The X-37B has now completed more than 600 days in orbit, with no indication when it will return to Earth.