The Super Guppy did not emerge from a clean drawing board. It is actually a mishmash of parts from several airplanes, along with a few custom pieces holding it all together. Some of those parts are from WWII-vintage designs. Despite its "Frankenplane" structure and relative age, the Super Guppy continues to do things that no other airplane in NASA's fleet can do. Indeed, few aircraft anywhere in the world can match this bulbous machine's ability to haul oversized cargo.
Before dissecting the makeup of NASA's current Super Guppy, it is worth reviewing the genealogy of aircraft that spawned it. As the story goes, aircraft salesman Lee Mansdorf and his friend, Jack Conroy conceived the "Guppy" idea in 1960 as an opportunity to provide logistical support to America's fledgling space program – even though NASA wasn't looking for help.

The manufacturers building spacecraft components were located all over the US. The only reasonable means to get these parts from one coast to the other was via ship travelling through the Panama Canal – an expensive and risky journey that could take weeks. Mansdorf and Conroy felt that air transport would be a much better method. Although there were airplanes capable of lifting the necessary weight, none were large enough to accommodate the girth of these loads. The industrious pair felt that they had a solution.

Mansdorf owned several newly-retired Boeing Model 377 Stratocruiser airliners. This 4-engined beast of burden (and its military doppelgangers, the C-97 Stratofreighter and KC-97 Stratotanker) were derivatives of the WWII-era B-29 bomber. In fact, all of these aircraft shared many parts, along with a later B-29 variant, the B-50. The fuselage design of the "Strato"-cousins resembles two cylinders with one stacked atop the other lengthwise. Mansdorf and Conroy envisioned replacing the upper cylinder with a much larger tube that could contain rocket parts and other plus-sized cargo.
Despite any real interest from NASA, the pair forged ahead using capital from private investors. They established Aero Spacelines as a company to manufacture and operate their planned fleet of overstretched planes. Their first example, the "Pregnant Guppy", was built using parts from two 377s. It emerged in 1962 measuring over 16 feet longer than its parts donors. More importantly, the upper section of the fuselage had grown to a diameter of 19 feet. This transformation gave the airplane its ungainly appearance, yet made it the only aircraft capable of carrying the 40 foot long, 18 foot diameter S-IV stage of the Saturn I rocket.

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http://www.tested.com/science/space/533738-nasas-super-guppyawkward-old-and-irreplaceable/