The North American XB-28: Too Much, Too Late

The sole XB-28 prototype. Note the remotely operated turrets.

With the North American B-25 Mitchell prototype (internal company designation NA-40B) already in the hands of the US Army Air Corps for flight testing in 1939, the promise of cabin pressurization offered a leap in bomber performance by being able to fly higher and faster. Accordingly, in August 1939, the Army issued the XC-214 specification which called for a pressurized medium bomber to supplant the medium bomber types that were soon to become operational. Only Martin and North American responded to XC-214. Martin's submission was for the XB-27 but the USAAC felt Martin didn't have a full grasp of the challenges of high altitude pressurization in their design and North American's submission, the XB-28 won the development contract. This took place on 15 November 1939 just three months after the Army issued its specification with North American inking a contract to begin formal design work on the XB-28. To give you an idea of the pace of development and the pressure of the looming clouds of war, the contract for the development of the XB-28 was signed around the same time that the Army ordered the B-25 Mitchell into production! The XB-28 had started out as a pressurized version of the B-25 Mitchell with a circular fuselage and Pratt & Whitney R-2800 Double Wasp engines with GE Type-C turbosuperchargers replacing the Wright R-2600 Twin Cyclone radials used on the B-25. Design work proceeded rapidly since the company was already at work on a pressurized successor to the B-25 at the time of the release of the XC-214 specification in August 1939. As design work progressed, changes were made stating with abandoning the Mitchell's twin fins for a single fin. Eventually the XB-28 as designed bore little resemblance to the Mitchell. The contract for three prototypes was signed on 13 February 1940. 

The five crew all sat in a pressurized compartment in the forward fuselage. 

Besides the change to more powerful R-2800 Double Wasp engines with GE turbosuperchargers, a third supercharger was also fitted to provide cabin pressurization. Heaters powered by gasoline warmed the air in the pressurization ducting for the pressure cabin located in the the forward fuselage. The elongated nacelles had an opening in the rear for the turbo-supercharger exhaust which added some forward propulsive power. The four bladed propellers were counter-rotating to cancel each other's torque to ease handling. Integral self-sealing fuel tanks took up most of the wings. Relatively unique for the day nose wheel steering was fitted and controlled by a lever in the cockpit similar to modern nose wheel tillers. Another unique feature was the use of fluorescent paint on the instrument panel and instruments that would glow at night from overhead UV lamps as an aid to night flying.

Overall configuration of the XB-28.

To simply the structure of a pressurized aircraft, the pressure cabin only occupied the forward fuselage. All the joints were sealed during assembly and the interior sprayed with a plastic sealant before installation of the cabin items. The cabin atmosphere was maintained at the equivalent of 8,000 feet up to an operating altitude of 33,000 feet. The crew of five was crammed into this space- with the pilot and co-pilot sitting side by side, behind them sat the primary gunner and the radio operator/secondary gunner. The bombardier/navigator sat in the nose compartment but could access the cramped flight deck via a floor panel by the co-pilot's feet. The bomb bay could carry up to 4,000 lbs of bombs and the three defensive turrets consisted of twin 50-caliber guns in dorsal, ventral and tail turrets that were operated remotely by the primary gunner and the radio operator/secondary gunner. Each gunner had a hemispheric observation window next to them and sighted the guns via a periscope system that protruded from streamlined twin fairings above and below the fuselage just aft of the flight deck. Initial plans for were for North American-designed turrets tied to a Sperry fire control system, but Sperry's resources were tied up with current production aircraft. It was decided to switch to General Electric for the remote fire control system and to have them be responsible for the turrets as well well. This imposed delays in the development as changes needed to be made to accommodate GE's equipment and systems. There was also a prevailing opinion at North American that Sperry's system was more advanced. A compromise was reached with the XB-28 defensive systems to use GE turrets and the Sperry sighting system. It's worth noting at this point that when the work on the remote turret fire control system on the XB-28 was under development, both Sperry and GE were working on getting the contract for the remote turret fire control system on the Boeing B-29 Superfortress- ironing out the kinks in the XB-28 gave GE valuable experience that helped the B-29 and made its system the production standard on the Superfortress.

Engine run up test at Mines Field in the summer of 1942.

The US entry into the Second World War slowed development of the XB-28 as priority shifted to production types and much of North American's resources were devoted to the production of the B-25 Mitchell and the P-51 Mustang. The maiden flight of the first prototype took place on 24 April 1942 at Mines Field (the site of today's Los Angeles International Airport/LAX) and the flight test program showed the XB-28 to be quite fast at high altitude, capable of 372 mph at 25,000 feet. Following the conclusion of North American's flight test program, the USAAF portion of the flight test program took place with the XB-28 operating out of Wright Field outside of Dayton, Ohio, for service trials. It was decided during the service trials that the third XB-28 prototype would be completed as a reconnaissance and photo-mapping aircraft designated XB-28A. North American was instructed to set aside work on the second XB-28 to get the XB-28A variant flying. The speed and altitude performance of the XB-28A was increased by reducing weight as well as installing more powerful versions of the R-2800 engines. The XB-28A made its maiden flight on 24 April 1943 (exactly one year after the first XB-28) but was unfortunately lost in flutter incident during dive testing on 4 August 1943 with the crew able to parachute to safety. At the time of the accident, design work on the production B-28 was underway with the most significant change being some extra scanning windows on the nose compartment for the bombardier/navigator.

By this point, however, it was realized that despite the outstanding performance of the XB-28, the realities of war showed that medium bombers already in service like the B-25 Mitchell and the Martin B-26 Marauder were most effective at low to medium altitudes flying interdiction missions where pressurization wasn't necessary. In the Pacific, B-25s equipped with extra forward firing machine guns were becoming very effective low level anti-shipping weapons while in the European theater, B-25s and B-26s operated most effectively at medium altitude (though some low level anti-shipping missions were flown in the Mediterranean against Axis vessels along the French and Italian coasts). The final nail in the XB-28's coffin was the Douglas product that was also first flown at Mines Field just a few months after the XB-28's maiden flight. The prototype Douglas XA-26 Invader first flew on 10 July 1942. It used the same engines as the XB-28, carried the same bomb load, but lacked pressurization which made it simpler to build and it only had a crew of three versus the crew of five on the XB-28. The sole XB-28 prototype was still at Wright Field at the time of the program's cancellation- it had its outer wings removed and sat out the war as a ground test article for pressurization tests before being scrapped.

Anigrand released a 1/72 resin kit of the XB-28 and this page has a great series of photos of a completed model that show the overall configuration of the XB-28. Take note of the hemispheric scanning bubbles on the upper lateral fuselage ahead of the wing for the gunners as well as the streamlined twin fairings above and below the forward fuselage for the sighting system to control the remote turrets.

Source: American Bomber Aircraft Development in World War 2 by Bill Norton. Midland Publishing, 2012, pp 66-69. Photos: USAF Museum, Anigrand

Grumman's Biggest Cat in World War II

In early 1941 the United States Navy issued a requirement to Grumman for development of a large twin-engined fighter powered by twin Pratt & Whitney R-2800 Double Wasp engines of 2,100 horsepower each. The Double Wasp was used on the Navy's other two fighters under development at the time, the Grumman F6F Hellcat and the Vought F4U Corsair- two such engines offered a tremendous leap in performance for what the Navy designated the F7F. The fighter would be heavily armed with four 20mm cannon in the wing roots and four 0.50-caliber machine guns in the nose with provision for carrying a torpedo under the fuselage or two 1,000 lb bombs under the wings. The large fighter would form the nucleus of the air wing of the new 45,000 ton Midway class aircraft carriers then under proposal. On 30 June 1941 the Navy authorized Grumman to begin work on the Tigercat. Two prototypes designated XF7F-1 were ordered on that day. As an interesting historical sidenote, five weeks earlier the US Army Air Force ordered two aircraft as well which were to be designated XP-65 but had lighter Wright R-1820 engines that would have been turbosupercharged to give the XP-65 even better peformance than the F7F. However, the differing specifications of the Navy and the Army were at odds as the USAAF wanted a lighter fighter than what the Navy desired. As a result, what might have been the first American joint-service fighter died quietly when the Army bowed out due to concerns about the design becoming overweight to meet Navy requirements. 

With the first F7F-1 Tigercats rolling off the Grumman production lines in April 1944, the Navy took the unusual step of having the Tigercat only operate with land-based squadrons of the US Marine Corps and skip  what promised to be a lengthy carrier qualification series of tests as the Navy had not previously operated large twin-engined fighters off its flight decks. The aircraft carrier it was intended for, the USS Midway, was still under construction at the time. At that point in the Pacific War the Navy felt that its fighter needs were being satisfied with the F6F Hellcat and the F4U Corsair. The Marines could operate the Tigercat as a fighter bomber which the Navy brass felt was more suited to the aircraft's design. Plans were set in motion to equip twelve Marine Corps fighter squadrons that were operating the Corsair at the time to re-equip with the Tigercat. The Corsairs would then be passed back to the Navy to meet fleet requirements in the Pacific. 

Production of the F7F-1 Tigercat came to sudden halt in December 1944 after only 35 aircraft were built. By this point in the war the Navy's carrier battle groups were gearing up for their first raids on the Japanese Home Islands themselves (Task Force 58 in February 1945) as a diversion for the assault on Iwo Jima. The Navy had a different role in mind for the Tigercat and the third F7F-1 was converted into the prototype XF7F-2N night fighter with an AN/APS-6 radar set in the nose, displacing the four 0.50-caliber machine guns, and a second seat for the radar operator over the wing behind the pilot. More powerful versions of the R-2800 radial were used to compensate for the increased weight of the second crewman and the additional radar equipment. However, changing requirements and technical issues meant that the F7F-2N was only an interim type, with only 65 being produced between December 1944 when F7F-1 production was stopped in favor of the night fighter and August 1945. Several of the -2Ns did go to sea, though, but for a series of comparison tests aboard the USS Antietam and the USS Shangri-La with the North American PBJ-1 Mitchell (the Navy version of the B-25 Mitchell). Both were being evaluated as night fighters by the Navy in April 1945 but it was soon found that both the Tigercat and the Mitchell were too large for the Essex-class fleet carriers and the Navy settled on radar-equipped variants of the F6F Hellcat and F4U Corsair instead. 

Only one Tigercat squadron made it to the Pacific before the end of the war with the Japanese surrender. Marine Corps squadron VMF(N)-533 was one of three Marine Corps night fighter squadrons established in 1943 operating the night fighter version of the Hellcat, the F6F-5N. Based in Okinawa at the time, the squadron was to convert to the night fighter Tigercat in theatre, but their new mounts arrived the day before the Japanese surrender. Patrols were made over China and Japan, but no operational engagements took place in the months following the Japanese surrender. Interestingly, the Royal Navy evaluated two F7F-2N Tigercats in the night fighter role, but decided to adopt the De Havilland Sea Hornet instead for the role. 

The most famous and most-produced version of the Tigercat, the F7F-3 and -3N, didn't arrive until March of 1946, too late for service in the Second World War. This variant was never carrier-qualified as well and by the time the Midway class carriers it was intended for were operational, the Navy's standard fighter was the Grumman F8F Bearcat and jets were just over the horizon. The F7F-4N was fully carrier capable and fully qualified unlike the previous versions, but only 14 of this model were built. In the Korean War, night fighting Tigercats in the hands of the Marine Corps finally drew blood and achieved fame that had eluded it during the Second World War. 

Source: United States Naval Fighters of World War II in Action by Michael O'Leary. Blandford Press, 1980, p136-141.