The space plane is to serve as a test platform for unspecified experiments and can stay in orbit for up to 270 days before gliding to an autonomous runway landing, the Air Force has said. The primary landing site is Vandenberg Air Force Base on the California coast northwest of Los Angeles.
The mission length wasn't disclosed by the Air Force.
The service has made public only a general description of the mission objectives: testing of guidance, navigation, control, thermal protection and autonomous operation in orbit, re-entry and landing.
However, the ultimate purpose of the X-37B and details about the craft have longed remained a mystery, though experts said the spacecraft was intended to speed up development of combat-support systems and weapons systems.
"This launch helps ensure that our warfighters will be provided the capabilities they need in the future," said Col. Andre Lovett, a launch official and vice commander of the Air Force's 45th Space Wing, in Thursday's statement.
The launch culminated the project's long and expensive journey from NASA to the Pentagon's research and development arm and then on to the secretive Air Force Rapid Capabilities Office. Hundreds of millions of dollars have been spent on the X-37 program, but the current total hasn't been released.
While the massive space shuttles have been likened to cargo-hauling trucks, the X-37B is more like a sports car, with the equivalent trunk capacity.
Built by Boeing Co.'s Phantom Works, the 11,000-pound craft is 9 1/2 feet tall and just over 29 feet long, with a wingspan of less than 15 feet. It has two angled tail fins rather than a single vertical stabilizer.
Unlike the shuttle, it was designed for launch like a satellite, housed in a fairing atop the expendable Atlas V rocket, and capable of deploying solar panels to provide electrical power in orbit.