The Cold War period (1947-1991), the showdown between East and West, saw tremendous technological growth with the advent of the jet age that followed the fighting of World War 2 (1939-1945). The turbojet supplanted the piston-driven powerplant in fighter and bomber development and the sky proved the limit for aerospace engineers from then on. Against this backdrop, companies hurried to develop faster-, higher-flying combat platforms to satisfy a new wave of over-battlefield requirements from their respective national leaders.
The strategic heavy bomber was one category of particular focus for both sides. This group was embodied during the World War 2 years by such four-engined types as the Avro "Lancaster" and Boeing B-17 "Flying Fortress", but the days of multi-engined, prop-driven bombers with now-modest war loads and machine gun defensive schemes was over - ahead lay new threats such as fast-closing interceptors and ground-based fire with Surface-to-Air Missile (SAM) systems still to come.
For CONVAIR, the resulting marriage between wartime competitors Consolidated Aircraft (makers of the classic B-24 "Liberator") and Vultee Aircraft, the B-36 "Peacemaker" made global headlines when it took to the skies for a first-flight on August 8th, 1946. It was subsequently adopted by the United States Air Force (USAF) in 1948 and led a relatively long service life into early-1959. Nearly 400 of the type were produced from 1946 until 1954, such was their value to the service (the bomber went on to make up the nuclear-capable arm for the vital Strategic Air Command branch of the USAF).
However, this bombing platform was a mix of technologies in that primary power stemmed from 4 x General Electric J47 turbojets (5,200lb thrust each) coupled with 6 x Pratt & Whitney R-4360-53 "Wasp Major" air-cooled radial engines offering an additional 3,800 horsepower of propulsion each. These were spread across wide-reaching wing mainplanes positioned at midships and gave the massive bombers its "legs" in the air. CONVAIR engineers were convinced that they could rewrite the design with a more effective, all-jet propulsion scheme seated within a more efficient swept-back wing mainplane offering inherent performance benefits and improved controlling.
The company approached USAF authorities with its planned bomber and March of 1951 saw the service commit its attention to a new CONVAIR design - designated "B-36G" as an offshoot of the B-36 bomber program line (mainly due to the cost-effective nature of transitioning an in-service airframe instead of developing a costly new one). Because changes to the original bomber form were to prove so extensive (only about 70% or so of the original bomber would be retained), the designation was soon changed to become "YB-40" so that the aircraft would head down its own development path as an all-new offering.
The new wing was set over the fuselage in typical fashion and considerable sweepback was apparent along both the leading and trailing edges. Under each wing were installed 8 x Pratt & Whitney J57-P-3 turbojet engines arranged in four pods (two engines to a pod) with each unit outputting 8,700lb of thrust. The fuselage remained largely faithful to the original B-36 save for a probe protruding from the nose and some length added to the aircraft.
Finalized dimensions included a running length of 171 feet, a wingspan of 206 feet, and a height of 60.5 feet. MTOW reached 300,000lb. Performance specs, as tested, went on to include a maximum speed of 508 miles-per-hour, a combat range out to 3,000 miles, a ferry range beyond 8,000 miles, a service ceiling up to 53,300 feet, and a rate-of-climb reaching 1,060 feet-per-minute.
Internally, there was to be an operating crew of five personnel made up of two pilots, a navigator, a communications man doubling as the bombardier, and another communications man doubling as the tail gunner. The only standard armament fitted would be 2 x 20mm trainable automatic cannons in a tail position to protect the bomber's vulnerable "six" against approaching interceptors. The rest of the armament suite would center around a bombload totaling 72,000lb of conventional or nuclear drop ordnance.
A first-flight of the completed YB-40 prototype was recorded on April 18th, 1952. However, the YB-40 languished in key areas, particularly against what became its period competitor - the Boeing YB-52. While carrying more bombs than the YB-52, the YB-40 suffered in performance and handling which resulted in a second place finish. Because of this outcome, the YB-40 was not developed beyond its flight test form and a second prototype lay partially completed when the program was abandoned in January of 1953. The USAF turned its attention to the YB-52 and adopted it as the B-52 "Stratofortress", never to look back, while the two YB-40 airframes were accepted by the service (to complete the terms of the deal) but quickly scrapped without much fanfare in 1954.
Thus ended the flying career of CONVAIR's newest "almost-bomber", the short-lived YB-40. Meanwhile the B-52 continues to fly as a frontline strategic heavy bomber for the USAF to this day (2019).