The Ultimate Superfortress: The B/RB-54A

Concept art of the B/RB-54A in flight
(Boeing Historical Archives)

During the Second World War Boeing worked extensively on further improvements to the B-29 Superfortress. The most important of these improved variants was the B-29D that involved swapping out the Wright R-3350 radial engines with the more powerful Pratt & Whitney R-4360 Wasp Major radial engine. In July 1945 the USAAF signed a contract for 200 B-29Ds, but with the end of the war and the rapid postwar demobilization, the B-29D contract was canceled. With the creation of an independent United States Air Force in 1947, there was a need for interim bombers pending the arrival of more advanced jet bombers. The USAF was already getting the Convair B-36 which took on the mantle of the heavy bomber, but the USAF also wanted the B-29D which would be redesignated as a medium bomber. The USAF had the B-29D redesignated as the B-50 to avoid the appearance of ordering a "wartime" bomber. Making its maiden flight on 25 June 1947, the B-50 Superfortress would eventually result in 320 examples of all variants produced.

Boeing, however, was working on an even more powerful and longer-ranged development of the B-50. Designated the B-50C, this evolution into the ultimate Superfortress was designed to extract as much speed and performance as was possible using a new version of the Pratt & Whitney R-4360 Wasp Major engine that added what was called a "variable discharge turbine" (VDT) to the engine. The standard Wasp Major used on the B-50 developed approximately 3,500 horsepower and a Wasp Major with a VDT could easily produce 4,000 horsepower, making it one of the most powerful production piston engines in the world.

The Wasp Major VDT
(from the Engine History website)

The VDT consisted of two General Electric CHM-2 turbosuperchargers that collected the hot exhaust gases from the 28 cylinders of the Wasp Major. A portion of the hot gases were diverted through an intercooler to provide turbosupercharging at high altitudes. The bulk of the hot gases went through the CHM-2 turbines and were exhausted out a variable area nozzle that resembled a set of eyelids. By adjusting the size of the nozzle, jet thrust could be achieved that had the potential to add as much as 15% to the speed of the B-50C over the production standard B-50. The Wasp Major VDT was already flying at this point on the Republic XF-12/XR-12 Rainbow long range reconnaissance aircraft. 

The scope of the changes needed to for the B/RB-54 resulted in a redesignation to B-54 with the planned reconnaissance variant being the RB-54. The jump in power output from the use of the Wasp Major VDT resulted in a redesign of the wings that resulted in a wingspan that was over 20 feet longer than that of the B-50 with a chord increase as well- an additional six feet of chord at the wing root and an additional four feet of chord at the wing tip. This provided additional fuel capacity along with external fuel tanks which were three times the capacity of the external tanks used on the B-50A on the outboard wings. The wingspan increase was so much that outrigger gears were needed under the outermost engine nacelles. Wind tunnel testing had shown that the new wing and powerful engine output also required a longer fuselage and the B/RB-54's fuselage was stretched 10 feet. Instead of the plexiglass domes used by the gunners on the B-29/B-50, low drag hemispheric sights were used. These used a fish eye hemispheric optical element that the gunner sighted through. Glenn's Computer Museum has some great pictures of the R/RB-54 hemispheric gunsight. The tail gunner also had a hemispheric gunsight but also had a radar to direct the four-gun turret as well which was mounted in fairing above the gun turret but below the hemispheric gunsight. Fairings were also present on the nose and under the forward fuselage for bombing and navigation radars. 

As a comparison, the B-29 weighed 120,000 lbs fully loaded and the B/RB-54 would weighed in at 207,000 lbs at takeoff. The Wright R-3350 engines of the B-29 developed 2,200 horsepower and the bomber had a range of approximately 3,250 miles. The B/RB-54 would have been able to push 8,000 miles of range. The mockups were completed in 1948 and the contract was signed for 43 bombers as an initial production lot. While the Secretary of the Air Force Stuart Symington and the USAF Chief of Staff General Hoyt Vandenberg were supportive of the B/RB-54 project, General Curtis LeMay, the head of the Strategic Air Command, felt that the B/RB-54 was inferior to the Convair B-36 Peacemaker particularly the B-36D that added four J47 jet engines under the outer wings. Pending the arrival of the B-52 Stratofortress, LeMay felt deterrence was better served by the B-36 which could fly faster, farther, higher, and carry a significantly larger bomb load. In the postwar atmosphere of austerity, more B-36s couldn't be accommodated in the Air Force budget and Secretary Symington offered LeMay more B-50s instead of increased numbers of B-36s. This was even more unsatisfactory to the outspoken SAC commander who then argued that if he couldn't get more B-36s, then the funding set aside for the B/RB-54 should be shifted over to get more of the Boeing B-47 Stratojet which made its first flight in December 1947. This was agreeable to all involved, even for Boeing as it meant more funding for the Stratojet program. The B/RB-54 project was cancelled with the prototype approximately 75% complete (it was converted from a B-50A) at Boeing's Seattle facilities. In addition, the addition of the outrigger gears wasn't popular with SAC as many of its bases would need widened taxiways and runways to accommodate the B/RB-54. 

The B-29 lineage would live on, though, in the C/KC-97 Stratofreighter (the last examples being retired in 1978) and in the commercial Boeing 377 Stratocruiser. But neither would have matched the leap in performance of the B/RB-54, the "ultimate" Superfortress.

The Retromechanix page has a series of superb photos via the National Archives that show the B/RB-54A mockup in detail as well as some schematic drawings. It's well worth the time to browse them!

Source: Boeing B-29 Superfortress (Crowood Aviation Series) by Steve Pace. The Crowood Press Ltd, 2003, p166-168. Boeing B-50 (Air Force Legends Number 215 by Geoffrey Hays. Ginter Books, 2012, pp 118-121.

The Massive Curtiss XP-71 Fighter

Early windtunnel model of the XP-71 with its wider twin-seat cockpit.

Before the entry of the United States into the Second World War, news stories were splashing headlines about the massed bomber attacks the Luftwaffe was conducting against British cities during the Blitz and in the run up to 1941, a concern about the potential of bomber attacks on the United States took root (despite the obvious shortcomings of any enemy bomber attack on US targets protected on each side by large oceans). A number of designs and studies were undertaken to evaluate the problem of intercepting enemy bombers and one of these resulted in a design specification for a large high-altitude fighter to carry a heavy cannon armament to attack bomber formations from ranges that would place it beyond a bomber's defensive guns. In April 1941, the Curtiss Aeroplane Company submitted six proposals with two, more refined, proposals the following November. One configuration met the proscribed needs of the military for a bomber destroyer and this aircraft was assigned the designation XP-71 with a $3.2 million contract for two prototypes which was signed on 28 October 1941. 

The Curtiss design had the internal company designation CW-29 and had it been built, it would have been the largest fighter aircraft ever developed in the United States- with a wingspan of 82.3 feet, a gross weight of 39,950 lbs and a fuel load up to 1,940 gallons, it was to be able to climb to 25,000 feet in just 12.5 minutes with a cruising speed of 428 mph. With an operating ceiling of 40,000 feet and a range of 3,000 miles, the XP-71 would attack enemy bomber formations well before they reached their targets. A heavy armament of two 37mm cannons with 60 rounds each and a 75mm cannon with 20 rounds was located in the nose and equipped with automatic feed systems so the aircraft only needed a minimal crew of just two. Two large Pratt & Whitney R-4360 Wasp Major 28-cylinder radial engines drove a total of nearly 7,000 horsepower to wing-mounted contra-rotating pusher props with a total of eight blades each and a diameter of 13.5 feet. General Electric turbosuperchargers would give the XP-71 the necessary high altitude performance along with a pressurized cockpit.

To put the massive size of the XP-71 into context, let's compare it with one of the larger production American fighters of the war, the Republic P-47 Thunderbolt. The XP-71 would have twice the wingspan, four times the gross weight, nearly four times the combat range, and nearly three times the engine horsepower of the Thunderbolt. In fact, the XP-71 had it been built would have been larger, heavier, faster, and longer ranged than even a late-model North American B-25 Mitchell medium bomber. 

This schematic hints at the immense complexity of the XP-71.

The mockup was inspected at Curtiss' St. Louis facility on 16 November 1942, at which time the design was revised from having two crew to just a single pilot. The original proposal had the two pilots sitting side-by-side. Following the mockup review, detailed design work took place well into 1943, by which time it was becoming clear to both the US Army Air Forces and Curtiss that the XP-71 might well be the most complex aircraft yet built. During ground firing trials of the nose cannon installation in February 1943, the nose structure failed in spectacular fashion, necessitating a redesign. Curtiss also had problems finding a suitable rangefinder for the fire control system and eventually settled on a radio-based model. Cooling of the R-4360 engines in the wing nacelles also required considerable attention with an annular intake with a gearbox-driven cooling fan being developed to insure adequate cooling airflow to the massive radial engines.

By the latter half of 1943 it was becoming apparent that the XP-71 was a plane without a mission as the strategic bombing campaign of Germany got underway. The likelihood of German bomber formations, let alone Japanese bomber formations, approaching US cities was almost nil. The USAAF considered re-roling the XP-71 as a photorecon aircraft, but no solid commitments were forthcoming. On 23 October 1943, the XP-71 program was terminated after the expenditure of $2.3 million with the first flight planned for June 1944. Curtiss attempted to salvage the program by pitching the XP-71 as an antishipping aircraft with its heavy nose cannons, but this role was already being filled by the proven B-25 Mitchell in the Pacific and Curtiss's engineering resources were needed on other projects.

Source: U.S. Experimental and Prototype Aircraft Projects: Fighters 1939-1945 by Bill Norton. Specialty Press, 2008, p138-139. Photos: National Museum of the United States Air Force.