This is the very first Globemaster III built, and initially wore a camouflage scheme, as shown in this site files photo of an early test flight. Subsequently it was assigned to the 412th Test Wing at Edwards AFB and the NASA caption to the above photo reads:

NASA Dryden is participating with the U.S. Air Force (USAF), other NASA centers, Boeing, and the Pratt & Whitney company in the Propulsion Health Management (PHM) portion of the Integrated Vehicle Health Management (IVHM) program. The program is using a USAF C-17A transport in an effort to enhance aircraft safety by enabling early electronic detection of potential problems with aircraft engines and associated systems.