CAPE CANAVERAL Fla. (Reuters) - The U.S. Air Force will take over two mothballed space shuttle processing hangars at the Kennedy Space Center in Florida for its secretive X-37B robotic spaceplane program, NASA said on Wednesday.

The agreement transfers two of the shuttle's three processing hangars for the military's X-37B Orbital Test Vehicles, built and managed by Boeing. Terms of the agreement were not disclosed.

Space Florida, a state-backed economic development agency, is contributing $9 million to refurbish the hangars for the X-37B program. Boeing contributed another $4.5 million. The upgrades are scheduled to be finished in December, NASA said.

The 29-foot (9-meter) long X-37B spaceplanes resemble miniature space shuttles. The Air Force currently has two vehicles, one of which has been in orbit since December 2012.

The military has not disclosed what the X-37B is doing in orbit, nor when or where it will land. Two prior X-37B missions lasted 224 days and 469 days respectively, and landed autonomously at Vandenberg Air Force Base in California.

The military previously said it was considering relocating the program to Florida to save money on operations. The vehicles are launched on unmanned Atlas 5 rockets from Cape Canaveral Air Force Station, located just south of NASA's Kennedy Space Center.

Landings also are now expected to take place on the Kennedy runway used by the now-retired shuttle fleet.

Boeing previously signed a lease with Space Florida for the third shuttle hangar, which the company will use for its NASA-backed CST-100 commercial passenger space taxis. The capsules are expected to begin flying crew to the International Space Station, a $100 billion research laboratory that flies about 260 miles (418 km) above Earth, in 2017.

(Reporting by Irene Klotz; Editing by Ken Wills)