43-52436, is a prototype heavy cargo aircraft built by Convair for the United States Air Force.

It was the largest piston-engined land-based transport aircraft ever built, and was developed from the Convair B-36 Peacemaker bomber, sharing the wings and some other structures with it.

However, the fuel and oil consumption of the six 3,500 hp (2,600 kW) Wasp Major radials powering the XC-99 and B-36 meant that the design was not economically viable, and the hoped-for turboprop powerplants did not materialize fast enough.

In 1951, the aircraft was retrofitted with 3,800hp Pratt & Whitney R-4360-53 engines, landing gear was strengthened and a cargo-loading system installed, including an internal elevator.

[5] The aircraft had continued to suffer additional corrosion during its years in Texas and was found to be in worse condition than expected, with the restoration task being beyond the resources of the museum in a realistic time scale.

[6] In an effort to preserve the aircraft for future restoration, the XC-99 was later moved incrementally to the 309th Aerospace Maintenance and Regeneration Group (309 AMARG) storage facility at Davis–Monthan Air Force Base, in Tucson, Arizona, where it will remain, in an area containing other aircraft belonging to the NMUSAF until the museum is able to restore it.

[5] In 2014, Lieutenant General John L. Hudson, USAF (Ret), director of the NMUSAF said that there would be insufficient resources for restoration for the foreseeable future.