The Boeing 377 Stratocruiser was airliner version of the 367 Boeing Stratofreighter, which in turn was the transport version of B-29 Superfortress. The Stratocruiser was simply a civil airliner version of the Stratofreighter. It first flew on July 8, 1947. It was generally considered the greatest airliner in the air before the coming of the 707 and was the flagship of the Pan Am fleet.
The Stratocruiser was typically used in first class transatlantic service (except for the United and Northwest planes), and other international routes. Many of these were converted to cargo operations, and several were used in the "Guppy" rebuildings, resulting in grossly outsized fuselages for hauling such things as rocket sections and airplane parts. Others became transports for the Israeli air force. All versions of the Stratocruiser featured a lower-deck lounge and bar, an innovation which, combined with a long range and high speed, made the Stratocruiser a very popular airliner.
During the early 1960s, Aero Space Lines ballooned the Stratocruiser's fuselage into a whale-like shape to carry spacecraft sections. Nine of the variants were assembled. The first was called the "Pregnant Guppy," followed by five larger "Superguppies" and three smaller "Miniguppies." In the process, one of the most elegant airplanes in the sky became one of the ugliest.