"The X-37B continues to demonstrate the importance of a reusable space plane," secretary of the Air Force Barbara Barrett said in a statement.
Altogether, the program, which has at least two of the reusable planes, has logged 2,865 days in space over the course of five missions, the Air Force said. The fifth mission launched on Sept. 7, 2017, aboard a SpaceX Falcon 9 rocket.
The Air Force plans to launch a sixth X-37B mission sometime in 2020.
The Boeing-built space planes resemble a smaller version of NASA's space shuttles and have a similar reentry trajectory that uses a runway, like the old shuttles. They feature a small payload bay and use a deployable solar array for power.
The 11,000-pound vehicle is about 29 feet long, with a wingspan of just under 15 feet, and was designed to stay in orbit for 270 days. It was originally a NASA program, with roots in the space agency's lifting-body research, that ran from 1999 to 2004. The X-37B is designed to serve as a platform for experiments and to offer insights on transporting satellite sensors and other equipment to and from space.