A secretive military spacecraft resembling a small space shuttle orbiter flew to Florida in the belly of a cargo plane this week to undergo final processing for launch on April 19.

The X-37B/OTV spacecraft undergoes final testing at Boeing. Credit: Air Force

The Air Force confirmed the critical preflight milestone in a response to written questions on Thursday.

The 29-foot-long, 15-foot-wide Orbital Test Vehicle arrived in Cape Canaveral, Fla., on Monday, according to the Air Force. The OTV spaceplane was built at a Boeing Phantom Works facility in Southern California.

Managed by the Air Force Rapid Capabilities Office, the OTV program is shrouded in secrecy, but military officials occasionally release information on the the spaceplane's progress.

"It is now undergoing spacecraft processing including checkout, fueling, and encapsulating in the 5-meter fairing of the Atlas 5 [rocket]," an Air Force spokesperson said.

The 11,000-pound vehicle will launch inside the nose cone of the Atlas 5 rocket. Liftoff is currently set for 10 p.m. EDT on April 19.

The reusable spacecraft is more famously known as the X-37B. The design is based on the orbital and re-entry demonstrator initially developed by NASA, then handed over to the Pentagon.

The NASA version of the X-37 featured an equipment bay 7 feet long and 4 feet in diameter for experiments and deployable payloads.

The Defense Advanced Research Projects Agency completed a series of approach and landing tests in 2007 using the White Knight airplane from Scaled Composites as a mothership.

It is easy to track the X-37's tumultuous history. NASA awarded the first X-37 contract to Boeing in July 1999, and the agency flew a series of visible atmopsheric tests on a scale model of the spaceplane in 2001. The X-37 began its transformation from a human spaceflight testbed to a military-run project when NASA shifted responsibility to DARPA in September 2004, a consequence of the space agency's new focus on lunar exploration.

Artist's concept of the X-37 under NASA management. Credit: Boeing Phantom Works

But specific payloads for the Air Force's OTV program aren't so clear. Officials have denied interview requests on the project, and the military only releases information through written responses.

The X-37B's mission is to "demonstrate a reliable, reusable, unmanned space test platform for the United States Air Force," the military fact sheet says. "Objectives of the OTV program include space experimentation, risk reduction and concept of operations development for reusable space vehicle technologies."

At the end of its mission, the X-37B will fire its engine and drop from orbit, autonomously navigating through a fiery re-entry on the way to its 15,000-foot-long primary runway at Vandenberg Air Force Base, Calif. Edwards Air Force Base is the backup landing site.

The duration of the spaceplane's first mission isn't being announced.

"The X-37B has the requirement to be on-orbit up to 270 days," the Air Force spokesperson said. "Actual length for the first mission will depend on the meeting the mission objectives, which consists of checkout and performance characteristics of the spacecraft systems."