The exact details of these experiments are a closely guarded secret. In a statement, the Air Force only revealed that the program "performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies."
This is the fifth X-37B space plane to be launched into orbit over the past decade, with each flight longer than its predecessor.
"Our team has been preparing for this event, and I am extremely proud to see their hard work and dedication culminate in today's safe and successful landing of the X-37B," said Brig. Gen. Doug Schiess in the statement released Sunday.
Each mission has been highly secretive, leading to public speculation that the planes could be used for spying activity or testing space weapons.
Here's what we do know about the X-37B: The craft is designed to test new navigation systems, as well as methods to reenter Earth's atmosphere and land safely.
The X-37B spacecraft is about 29 feet (8.8 meters) long and 9.5 feet (2.9 meters) high, with a wingspan of nearly 15 feet (4.5 meters).
The planes also have a unique design. They launch into orbit atop powerful rockets and then break away to carry out their mission. When they return to Earth, they touch down horizontally on a runway, like a commercial airplane or space shuttle orbiter coming in for landing.
The plane that just landed was launched to space in September 2017 on a SpaceX Falcon 9 rocket. In its Sunday press release, the Air Force announced it was preparing to launch the sixth X-37B mission in 2020.