One of the criticisms of the B-36A was that it was already obsolete before it had taken off on its first flight, and did not have the performance necessary to survive in hostile airspace against determined fighter attack. The B-36C was an attempt to cure some of the performance problems encountered by the B-36A.
The B-36C was to be powered by six 4300 hp Pratt & Whitney R-4360-51 variable discharge turbine (VDT, or turbocompound) engines. In the VDT engine, exhaust gases from the piston engine would pass through a General Electric CHM-2 turbosupercharger which featured a clamshell nozzle that could create a jet thrust by varying the size of the turbine exit. The variable discharge nozzle was to be operated by automatic control activated by a manifold pressure sensor.
Convair claimed that the VDT engine would give the B-36 a top speed of 410 mph, a 45,000 foot service ceiling, and a 10,000-mile range with a 10,100-pound bombload. Unfortunately, the use of these engines would require a change from a pusher to a tractor format, which would in turn require a major redesign of the entire aircraft.
The VDT engine had originally been proposed for a version of the Boeing B-50 four-engined bomber, which had been assigned the designation XB-54. In mid-1947, Convair proposed that 34 aircraft out of the original hundred that were ordered be completed as B-36Cs. Serials would be 44-92065/92098. To offset the cost of converting one B-36 to a prototype B-36C configuration, Convair suggested that three B-36s be cut from the current procurement contract. This project was approved by General Spaatz, Chief of Staff, in July of 1947.
Many people both inside and outside the USAF thought that the B-36 was already obsolete, and believed that fast jet bombers should be acquired instead. However, jet bombers were at that time still many years away, and in any case promised to have a much shorter range than the B-36. Others wanted to use the B-36 as an all-purpose bomber, one that would be capable of delivering a wide range of conventional as well as nuclear ordnance in tactical as well as strategic missions. Yet others favored the B-50 over the B-36 because of its higher performance and also because it was considerably cheaper. After much discussion, it was decided to retain the B-36 as a special-purpose bomber that would be used strictly for long-range nuclear attack, with the hope that the fleet of B-36s would eventually be replaced by the B-52 when it became available. Consequently, it was concluded that there was no real need for a VDT-equipped B-36 and that the retrofit with VDT engines would serve no purpose other than to delay the completion of the 100 B-36s already on order and to drive up costs still further. The VDT-powered B-36C prototype was cancelled on August 22, 1947.
The cancellation of the prototype did not stop Convair from going ahead and proposing on September 4, 1947 that the last 34 B-36s in the hundred-plane contract be completed as B-36Cs. Convair proposed that the extra cost of the production of the 34 B-36Cs could be met by reducing the overall order to only 95 B-36s. Convair claimed that the B-36Cs could be produced without delaying the current contract by any more than 6 months. It was even suggested that the remaining B-36A and B aircraft on the contract could be retrofitted to B-36C standards.
The Convair proposal was accepted at least in principle on December 5, 1947. However, the decision on whether to retrofit the 61 remaining B-36s as B-36Cs was deferred until later. As it turned out, the attempt to mate the VDT engine with the B-36 airframe failed completely. There were problems with the engine cooling requirements generated by the aircraft's high operating altitude, which degraded the engine's performance. The drop-off in engine power at high altitude made Convair's estimates for the performance of the B-36C completely unrealistic. The project was quietly dropped in early 1948, and 44-92065/92098 were completed as standard B-36Bs.