The US Air Force's mysterious X-37B space plane returned to Earth on Sunday after completing its fifth secret space mission that lasted more than two years.

X-37B Orbital Test Vehicle successfully landed at NASA's Kennedy Space Center Shuttle Landing Facility at 3:51 a.m. US Eastern Time, wrapping up its on-orbit experiments for 780 days, which broke its own record set in the fourth mission.

The X-37B, built by Boeing and initially designed for an on-orbit duration of 270 days, looks much like NASA's retired space shuttles, only much smaller. It is boosted into orbit by a rocket and lands like an aircraft on a conventional runway. SpaceX's Falcon 9 rocket carried the space plane to orbit on Sept. 7, 2017.

"This program continues to push the envelope as the world's only reusable space vehicle. With a successful landing today, the X-37B completed its longest flight to date and successfully completed all mission objectives," said Randy Walden, US Air Force Rapid Capabilities Office director.

"This mission carries small satellite ride shares and will demonstrate greater opportunities for rapid space access and on-orbit testing of emerging space technologies," the US Air Force said in 2017.

However, the exact purpose of the X-37B was never clearly revealed and the secrecy surrounding the project has led to speculation that the solar-powered X-37B can be used as a spy satellite or to deliver weapons from space.

The US Air Force is preparing to launch the sixth X-37B mission in 2020.