The Iconic XC-99

Author's note: This article appeared in the March 2003 issue of “Aviation History” magazine. Since then, the status of his historic aircraft has changed. In approximately 2011, the curators of the National Museum of the US Air Force decided, because of funding and manpower issues, to suspend restoration of the XC-99. Instead, they moved it to the Davis-Monthan AFB "boneyard" near Tuscon, Arizona, for safer storage in the arid desert environment. If and when it will eventually be restored is still in question.

On the Sunday morning of November 23, 1947, the largest land transport aircraft the world had ever seen took to the air.  An estimated 90,000 spectators lined the streets and hillsides surrounding San Diego’s Lindbergh Field, as the 133-ton Convair XC-99 broke ground.  This flight, lasting just over an hour, began an eventful decade of service for what was to become one of the most visible one-of-a-kind airplanes since the Spirit of St. Louis.

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The XC-99 was little more than an experimental modification of the magnificent U.S. Air Force B-36 intercontinental strategic bomber.  The giant B-36 was the last of the great piston-engine, propeller-driven bombers to materialize from WWII-era drafting boards.  Paradoxically nicknamed the “Peacemaker” the six-engine B-36 had the important distinction of being the first bomber with sufficient range to hit virtually any target in the world. As such, it was destined to become a major player in the new Cold War that succeeded World War II.

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The idea behind the XC-99 originated at Consolidated Aircraft Corporation in May 1942 as a commercial land plane study.  Engineers envisioned this new B-36-based design as a precursor to the newest fleet of Air Force cargo/troop carriers, as well as the first of a new line of commercial super transports.

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To cut costs, Consolidated, who later merged with Vultee Aircraft to become Convair, incorporated the wings, engines, tail and landing gear of the B-36 into the new transport. In fact, approximately 60% of the XC-99’s parts were to be interchangeable with the B-36.  The only obvious difference between the two aircraft was the replacement of the existing sleek B-36 fuselage with a much larger one consisting of two separate decks.  Finally, on May 26, 1949, after years of development and many modifications, Convair handed its new Goliath of the air over to the Air Force for evaluation. 

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Like the B-36, the XC-99 was powered by six 3,000 hp Pratt and Whitney R4360 Wasp Major radial engines, distinctively mounted on the trailing edge of the wings in pusher configuration.  As an added feature, the nineteen-foot Curtiss Electric propellers were fully reversible, giving the XC-99 the unique ability to actually back right up to its unloaded cargo, like a flying Mac truck!

A B-36 and the XC-99 (right) in flight. (USAF)

The 230-foot wingspan of the giant transport was identical to that of the B-36, while its 182.5-foot fuselage was almost 20 feet longer.  The vertical fin, ten feet taller than the B-36, was a towering 57.5 feet tall – as high as a five-story building.  Not surprisingly, such lofty dimensions created serious maintenance challenges.  In order simply to gain access to the vertical fin and rudder, for example, it was necessary to first build a skyscraper-sized wooden scaffolding structure!

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Not only was the XC-99 fuselage longer than the B-36, it was also broader.  At 20 feet in diameter, it equaled a modern DC-10 fuselage, making the XC-99 one of aviation's first wide body aircraft. It contained two separate decks with a total volume equivalent to that of ten railroad freight cars and a usable cargo space of some 16,000 cubic feet.  Such spacious storage capacity, supported by almost 5000 square feet of wing area and the equivalent of five locomotives worth of thrust, made it possible for the XC-99 to carry loads never before thought possible in an air transport.  Having a useful load of more than 50 tons, it could alternatively carry 400 fully equipped combat troops, or more than 300 litter patients complete with a full complement of medical attendants.

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The original versions of both the XC-99 and the B-36 featured a main landing gear consisting of two massive single tires.   Each of these 1,320-pound Goodyears had a whopping nine-foot diameter and width of three feet, making them the largest ever before manufactured for an airplane.

It didn’t take long for these huge single tires to prove their impracticality.   Both of these aircraft were so heavy when fully loaded that the single tires exerted enough pressure to crush through the pavement of almost any runway in existence.   In fact, only three airfields in the entire country had specially reinforced concrete runways capable of accommodating these aerial giants.   This serious limitation prompted designers to retrofit both the XC-99 and B-36 with smaller four-wheeled main landing gear bogies.  These spread the weight over a wider area and greatly expanded the number of airfields these aircraft could safely use.

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The flight crew of the XC-99 typically consisted of the pilot, copilot, navigator, radio operator and two flight engineers.   In addition two scanners were also routinely included, whose job it was to advise the pilot about engine performance, flaps and landing gear operation.   None of these functions were visible from the flight deck.

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For long flights, a relief crew also went along.   These off-duty crewmen traveled in relative luxury, having at their disposal bunks and a galley equipped with hot plates, an electric oven, icebox, dining table, chairs and food storage compartments.   In addition, there was hot and cold running water, an electrical incinerating toilet and even a private compartment for the aircraft commander.

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The XC-99 boasted a maximum fuel capacity of 21,116 gallons, giving it an unmatched range of 8,100 miles with a limited load of 10,000 pounds.   With a full 100,000-pound load, it could still fly an impressive 1,720 miles at just over 300 mph.

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Amazingly, the XC-99’s huge control surfaces – almost as large in area as the entire wing of a B-24 bomber – were operated without any power boost.   Instead, the controls were linked to smaller trim tabs that aerodynamically moved the larger control surfaces.   Although understandably somewhat sluggish, the aircraft reportedly handled well in spite of its size and its unconventional control mechanism.

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The XC-99 established its place in aviation history as it broke almost two dozen international records for cargo hauling and distance.   It amazed the world for almost a decade by airlifting massive loads thousands of miles nonstop.   Everywhere it landed, the aerial giant was the object of attention, and it was a regular feature both in the press and on the air.

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As a consequence of its far-reaching travels and imposing appearance, the much-celebrated XC-99 became a sort of ambassador of the air for the Air Force.   It was a source of great pride for Americans, and equally important, a highly visible symbol of U.S. military dominance during a period of high international tension.

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The civilian transport version of the XC-99 never made it off the drawing board.   The massive double-deck fuselage of the so-called Model 37 was designed to carry 204 passengers plus more than seven tons of cargo.   It was anticipated that customers would pay well to travel luxuriously between New York and London in only nine hours.   Pan American World Airways was so impressed that it ordered 15 of these “Super Clippers”  in February 1945.   They later cancelled this order, but kept an option open on three aircraft.

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When the intended engine for the Model 37 – a 5,000 hp gas turbine –failed to become available, the airline was forced to cancel the order altogether.   The operating costs of the existing 3,500 hp radials were too great to allow the proposed airliner to be flown profitably.   Besides, it was becoming apparent that post-war airline demands would not support the use of such a large capacity aircraft.   This ended any prospect for a Convair “Super Clipper.”

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Almost inexplicably, the Air Force also declined to order additional copies of the XC-99, in spite of its many aeronautical successes.   It could carry more cargo farther and cheaper – at 16 cents per ton-mile – than any other military transport in existence at the time.   Because of this, the Air Force operated its single XC-99 longer and more extensively than any other original prototype – 7,400 flight hours and almost eight years. Why then after years of outstanding service did no orders for additional aircraft materialize?

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Probably the biggest enemy of the XC-99 was timing.   The era of piston-engine, propeller-driven aircraft was nearly at its end by the time the Air Force accepted the XC-99 in 1949.   Some argued that even the B-36 was obsolete from the outset and should have been scrapped in favor of the newer and faster generation of jet bombers, such as the B-47.   Thus, a large investment in the relatively slow piston-engine transport was just not in the best interests of the Air Force.

Another factor that worked against a production run for the XC-99 were its high maintenance and operating costs.   Although the large Convair could haul 50 tons of freight relatively economically, it was often difficult to amass enough high priority cargo to make a flight cost-effective.   This became more of a problem with the decreased cargo demands that followed the end of the Korean War.   The phasing out of the B-36 bomber in the late 1950s exacerbated this problem.   Since a major function of the XC-99 over its service life had been the airlifting of B-36 engines, this important mission disappeared with the demise of the big bomber.

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In March 1957 the Air Force finally retired its aging XC-99 – the only one of its kind ever built – after it became apparent that such a large and ungainly air transport was no longer practical.   Initially, the U.S. Air Force Museum expressed an interest in acquiring the XC-99.   Unfortunately, the cost of rendering the worn out old transport airworthy for the flight from Kelly AFB in San Antonio, Texas, to the museum at Dayton was prohibitively high.   So instead, it was sold as surplus and put on display in San Antonio.

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Over the years, the grounded giant steadily deteriorated as it passed hands several times. It gradually became a derelict, as it continued to languish in a vacant field a few yards from the runway at Kelly AFB.   Finally, in 1993 the neglected XC-99 was acquired by the Kelly Field Heritage Foundation for $65,000 and moved back on base, where cosmetic repairs were begun.   When Kelly AFB – itself a landmark of U.S. military aviation heritage as the oldest continuously active military airfield in the nation – was closed in 1999, the tired old transport was forced back into “storage.”   Once again, the XC-99 faced an uncertain future.

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Fortunately, the story of the XC-99 does not end here.   The National Museum recently took possession of the giant old aircraft.   They dismantled it and shipped it piece by piece from San Antonio to Dayton, where it is in the process of being reassembled, restored and eventually placed on display.   This is a project that undoubtedly will take considerable time and resources, but it is certainly a positive step towards preserving this important piece of our aviation heritage.

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During its nearly eight years of active service, XC-99 #43-52436 accumulated a total of 7,400 flight hours, traveled nearly 1.5 million miles – the equivalent of 59 times around the world – and in the process, delivered 60 million pounds of valuable cargo.   Perhaps more importantly, it served as a classroom to our nation’s Air Force.   Vital lessons were learned about how to accumulate, load and move tons of freight via super-sized aerial transport, as well as how best to design the airplane to efficiently carry this cargo.   This information would prove invaluable in the years to come.

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The XC-99 had finally outlived its usefulness, but it clearly had demonstrated that operating such a large aircraft was not only possible, but also practical, and even economical – especially during wartime, when the need for rapid movement of men and materiel is greatest.   It is therefore not quite as ironic as it might seem, that in the wake of the demise of the XC-99 were planted the seeds of even more massive aircraft for the future.   The development of such huge transports as the C-5A Galaxy and Boeing 747 were already on the horizon.   Indeed, the singular XC-99, more than any other aircraft in history, cleared the path for today’s era of giant transport aircraft.