The X-37B mission is managed by the Air Force Rapid Capabilities Office, a unit inside the Air Force that specializes in developing aircraft technology. It grew out of a project at NASA in 1998 aimed at reducing the cost of space transportation, It was transferred to the Pentagon’s Defense Advanced Research Projects Agency, or DARPA, in 2004, where it became instantly classified. The X in front of the program’s name means the spacecraft is a member of a long line of planes and rockets the U.S. has developed for experimental purposes, usually with a good amount of secrecy. The X-37B, built by Boeing, borrowed its aerodynamic design from the NASA space shuttles that ferried astronauts to and from space for 30 years, but it uses a different kind of heat shield. The X-37B weighs about 11,000 pounds at launch and has a wingspan of 14 feet. It can spend 270 days or more in orbit, and is launched into space aboard an Atlas V rocket.
The Air Force only made public two tests involved in this latest mission: an electric-propulsion device that could allow spacecraft to carry heavier payloads and a NASA experiment aimed at exposing 100 different kinds of materials samples to microgravity. This is where official information from the Air Force usually stops, and the speculation begins. Some in the military-space community believe the military might be tinkering with advanced surveillance sensors, or testing the electric-propulsion devices, known as thrusters, so it can put future reconnaissance satellites in lower orbits, where they can better see targets on the ground. Others say the military wants to use the spaceplane as a weapon capable of approaching other satellites to observe them and, perhaps, interfere.
https://www.theatlantic.com/technology/archive/2017/05/why-so-secretive/525969/