How post-war ambition, German research, and jet technology shaped the aircraft that would carry America’s nuclear deterrent
At the close of the Second World War, the Boeing B-29 Superfortress stood as the most capable strategic bomber in existence—yet even it was constrained by range. Operating from mainland China in 1944, it could carry only a limited payload; its full potential was only realized once the Americans established bases in the Marianas.
As early as April 1941, the USAAF had issued a specification for an advanced bomber capable of carrying a 10,000-pound payload over 5,000 miles, or 72,000 pounds over shorter distances, with a top speed of 300–400 mph at 35,000 feet. Two very different companies answered the call.
Convair and the B-36
Convair proposed a massive, six-engine bomber with a 230-foot wingspan, designated XB-36. A contract for two prototypes was signed on 15 November 1941. Development was slow initially, but it accelerated in 1943, when strategic bombing appeared to be the only viable way to strike Japan’s home islands. An order for 190 production aircraft followed in July that year. The prototype flew on 8 August 1946, and a total of 154 B-36s were eventually built before production ended in 1954. The type served as Strategic Air Command’s primary nuclear delivery platform until the Boeing B-47 entered service.
Northrop and the Flying Wing
Northrop took a radically different approach, drawing on John Northrop’s flying-wing research dating back to 1927. An early test aircraft, the NIM, completed over 200 flights at Muroc by September 1941, encouraging Northrop to submit a four-engined long-range bomber design. A prototype was ordered under the designation XB-35 in November 1941 — one week after the Convair order.
Design work began in early 1942, with Glenn L. Martin Company handling wing design and engine installation, while Northrop managed aerodynamics, undercarriage, and control systems. Four scaled models (the N-9M series, with 60-foot wingspans) were built to test flight characteristics and train future pilots.
Construction of the XB-35 prototype began in January 1943. Orders followed for thirteen pre-series YB-35s and, eventually, 200 production B-35As — though the latter were cancelled as jet propulsion entered operational service. On 25 June 1946, test pilot Max Stanley flew the piston-engined XB-35 from Hawthorne to Muroc.
The aircraft was an imposing machine: a 172-foot all-metal wing with a 37-foot root chord, a swept leading edge, hydraulically assisted controls, and automatic leading-edge slats. A pressurized fuselage nacelle housed a nine-man crew. Defensively, the aircraft carried up to twenty .5-inch machine guns in six turrets. Power came from four 18-cylinder Pratt & Whitney Wasp Major engines driving coaxial eight-bladed propellers. Empty weight was 90,500 lbs, with a maximum overload of 212,544 lbs. Estimated top speed was 393 mph at 34,650 feet, with a range of 8,125 miles—equivalent to a non-stop New York–Paris round trip carrying a 16,000-lb bomb load.
Testing was troubled from the start. Propeller governing mechanisms and engine reduction gears caused persistent problems, forcing a suspension of flights until simplified four-bladed propellers could be developed.
The Jet Conversion: YB-49
In 1947, the second XB-35 and the first pre-series YB-35 arrived at Muroc. The second pre-series aircraft (42-102367) flew on 21 October that year and was subsequently converted to eight Allison J35-A-5 turbojets—initially called the YB-35B, then redesignated YB-49. The jets were grouped in two clusters of four, flanked by vertical fins to improve directional stability. The crew was reduced to seven, and the defensive armament to a single rear-mounted twin-gun turret. Loaded weight was 216,600 lbs, with a typical bomb load of 10,000 lbs over a range of 2,800 miles.
Although the YB-49 achieved 520 mph — well above the piston-engined XB-35’s 393 mph — its range was roughly half that of its predecessor, and an inherent instability made it a poor weapons release platform. Flight testing quickly demonstrated its unsuitability as a strategic bomber. During one endurance run, however, a YB-49 remained airborne for 9 hours 30 minutes, covering 3,458 miles at an average speed of 382 mph between 35,000 and 40,000 feet, suggesting potential in the reconnaissance role.
In February 1948, a decision was made to produce thirty reconnaissance variants, the RB-49A, with nine YB-35s to be converted to YRB-49A standard by Convair. The order was cancelled in April 1949, though one YRB-49A—fitted with six Allison J35-A-19 turbojets, four buried in the wing and two in underwing pods — was completed and flew from Hawthorne to Edwards AFB on 4 May 1950.
The YB-49 program ended catastrophically. The first aircraft exploded without warning over Muroc on 5 June 1949, killing all five crew members; the second was destroyed under identical circumstances on 15 March 1950. Before its loss, one YB-49 had appeared in the film adaptation of H.G. Wells’ The War of the Worlds, dropping an atomic bomb on the invading Martians.
A further variant, the EB-35B, was planned around Northrop’s own Turbodyne XT-37 turboprop—the first American turboprop bench-tested with a propeller fitted, in December 1944. Development continued through various corporate restructurings until the project was abandoned following unforeseen technical failures. The EB-35B never flew.
By late 1950, only the YRB-49A remained. It was scrapped in October 1951. The Convair B-36, by then, had been in SAC service for four years.
Northrop’s flying-wing work did not end entirely there. In 1948, the company built the small X-4 Bantam jet research aircraft, powered by two Westinghouse J30 turbojets, which reached Mach 0.95 in a shallow dive. Operated by NACA, the X-4’s data on swept-wing tailless aircraft informed the design of operational types, including the Chance Vought F7U Cutlass and Douglas F4D Skyray.
Boeing and Convair: The Final Contest
In April 1945, the USAF issued a requirement for a jet-powered heavy bomber to replace the B-36. Boeing, drawing on its B-17/B-29 lineage, initially proposed a turboprop swept-wing design before revising it to eight turbojets. The XB-52 began ground testing on 29 November 1951; the YB-52 flew first on 15 April 1952.
Convair’s response was a swept-wing evolution of the B-36, originally designated YB-36G and later renamed YB-60. Two prototypes were ordered on 15 March 1951; the first flew on 18 April 1952, powered by eight Pratt & Whitney J57-P-3 turbojets in paired underwing pods. Three-quarters of the structure was shared with the B-36, and the planned production version would have reached 550 mph at 55,000 feet. The Boeing B-52, however, had flown three days earlier and proved superior on almost every count. The YB-60 program was cancelled, and both prototypes were eventually withdrawn from use after serving in research and development work.
This article is from the book “Combat Aircraft Prototypes since 1945” by Robert Jackson, available here.
