Although the Boeing 707 is known worldwide as the machine which took civil aviation from the piston engine era into that of the jet engine, what is very often not known is that its existence was only made possible by the success of its immediate predecessor, the KC - 135, a flying fuel tank used for refueling the strategic B - 52 bomber, also made by Boeing. Although these two models came from the same prototype, the "Dash 80", which first flew in July 1954, they were in fact two radically different machines sharing only a limited number of common features. More than 800 KC - 135s were produced spawning an impressive number of variants and specialized versions, from training astronauts to collecting samples, from transporting headquarters staff to waging electronic warfare. More than 1000 Boeing 707s were built up to the end of the 20th century and also had a long career with various versions and re-engined variants, the last machines coming off the production lines, so the story goes, destined for the military market, in the form of the E - 3 Sentry which will remain in service into the middle of the present century.