Excerpt from nbcnews.com

No seems to know much about the Air Force’s X-37B secret plane except that it to be working exactly as designed. The unmanned Boeing-built craft, which resembles a miniature shuttle, returned to Earth on Friday after nearly two years — 674 days, to be exact — in . It’s the X-37B program’s mission to and by far the longest.
The plane landed at 9:24 a.m. local time on Oct. 17 at Vandenberg Air Force in , the Air Force’s 30th Space Wing announced. 
“The 30th Space Wing and our mission partners, Air Force Rapid Capabilities Office, Boeing, and our base support contractors, have put countless hours of hard work into preparing for this and today we were able to see the culmination of that dedication,” Colonel Keith Balts, 30th Space Wing commander, said in a release. “I’m extremely proud of our team for coming together to execute this third safe and successful . from our on console space operators to our airfield managers and civil engineers take pride in this unique mission and exemplify excellence during its execution.” 
But just what did the X-37B do up there? Officially, the Air Force isn’t telling. 
What is known is that the X-37B has no human pilot, or at least not one in its windowless cockpit. It’s operated remotely and on its own.
The plane’s size there isn’t room on for much except avionics equipment, fuel for the thrusters, and a mysterious cavity about the size of a truck bed that could contain all manner of sensors, experiments, hardware — perhaps some bacterial colonies, or a bomb. No one can be sure what’s inside.