VC-137 "Stratoliner" is the designation used by the United States Air Force (USAF) to mark its Boeing 707s configured to the VIP passenger transport role. The Boeing 707 was developed in the latter part of the 1950s, first flying in December of 1957 and, since then, the model series has been produced in over 1,000 examples into 1979. The VC-137 remains in active service as of this writing (March 2014) and has seen several designations emerge since its formal adoption. Outwardly, the VC-137 retains the same general form and function as the original 707. The first VC-137 was delivered in 1959 and procurement proved limited.
VC-137A were the original early acquired marks and numbered three examples. These were internally configured to seat twenty-two in comfort with extra equipment to serve occupants as an Airborne Command Post (ACP). Power was served through 4 x Pratt & Whitney J57 turbojet engines. These aircraft were then redesignated as VC-137B. The VC-137B was refitted with 4 x Pratt & Whitney JT3D-3 turbofan engines and they, themselves, would later be known under the C-137B designation.
Beginning in 1961, the USAF procured an additional Boeing 707 aircraft (707-353B) and assigned it the VC-137C designation to serve the President of the United States and staff members. A second example followed in 1972 and, as with the earlier VC-137Bs, the VC-137C became known as C-137C.
The EC-137D designation was reserved for related Early Warning and Control System (EWAC) aircraft prototypes and numbered two. These then evolved under the E-3A "Sentry" designation to become their own notable Boeing 707 offshoot, serving with several world powers including NATO.