In 1947, Convair rolled out an aircraft so large that, three-quarters of a century later, no piston-engined land transport has ever exceeded it. The XC-99 could swallow a hundred thousand pounds of cargo or four hundred troops on its two full decks, and haul them across a continent. Exactly one was ever built.

It was, in effect, a bomber that had swapped its bombs for cargo — and grown enormous in the process.

A bomber turned freighter

The XC-99 was not designed from a blank sheet. Convair took the wings, the six huge rear-facing pusher engines, the tail and the landing gear of its B-36 Peacemaker intercontinental bomber and married them to an entirely new, deep, double-deck fuselage. The result shared the B-36’s reach but traded its bomb bay for a cavernous hold.

Operation Elephant

After its first flight in San Diego in November 1947, the XC-99 was handed to the Air Force in 1949. Its working life began in earnest in July 1950 with a mission fittingly codenamed Operation Elephant: it carried 101,266 pounds of cargo — engines and propellers for the B-36 fleet — from San Diego to Kelly Air Force Base in Texas. It would later lift 104,000 pounds in a single load.

Through the early 1950s it flew logistics runs across the United States and even across the Atlantic, moving the kind of outsized loads that would otherwise have gone by ship.

Too big to keep

For all its capacity, the XC-99 was a one-off, and a thirsty, complex one. Keeping a single unique giant airworthy was expensive, and by 1957 the Air Force retired it. The jet age was arriving, and turbine-powered transports would soon do the job faster and more cheaply.

The lone airframe survived years in the open at a Texas base and later in the Arizona desert before being dismantled and moved to the National Museum of the U.S. Air Force, where it awaits restoration — the only one of its kind there ever was.

Sources: National Museum of the U.S. Air Force; Vintage Aviation News; PlaneHistoria; Wikipedia.