The
Boeing 707 survives to this day in that ultimate collection of slightly outdated aircraft - the
U.S. Military! The U.S. Air Force's most popular 'utility'
airframe for large jet applications is the '-135' body, as in the
KC-135, the
RC-135 (
Rivet Joint), and even the
VC-135. All -135 airframes are essentially militarized 707s. As the aircraft was retired from commercial roles, the U.S. military picked up some low-hour examples for spares; in addition, they have re-engined a number of different -135 aircraft with more efficient
turbofan engines.
A minor piece of trivia; when the 707 was unveiled to the press and public, Tex Johnston, the test pilot flying the demonstration aircraft inverted the aircraft for several seconds (against orders!). The press and industry were impressed as hell.
Minor correction: The -135s aren't technically militarized 707s. The 707 and the -135 are both descended from the same prototype - the Boeing 367-80, or 'Dash 80' mentioned in kermitov's writeup above. That prototype was intended to launch the 707 and the entire 7-- family, but was named the '367-80' to hide its true nature from industrial espionage during development. It first flew in 1954, with performance specifications that are quite similar to today's jetliners in every area except perhaps noise level and fuel consumption due to its use of turbojets rather than turbofans. The Dash 80 was used to secure orders for both KC-135s and 707s, and today it (the sole Dash 80 prototype) rests at the U.S.'s National Air and Space Musuem's Steven F. Udvar Hazy Center at Dulles International Airport.