STRUCTURE

The B‑29’s structure was unique among World War II bombers, not just in terms of its size, but also in the elements of its construction. The aircraft’s aerodynamics were unprecedented, it had pressurized cabins, an innovative control system and tricycle landing gear. The B‑29 represented the leading edge of aeronautical engineering in the mid-1940s.

Boeing had paid unusual attention to the design’s aerodynamics. The fuselage, wings and appendages were designed in such a way that drag was kept to a minimum. Protrusions, including the gun turrets, were reduced in size and streamlined. Furthermore, skin panels were butt-seamed and the skin attached with countersunk flush-head rivets. The fuselage cross-section was circular throughout, with no discontinuities. It had a cylindrical body that tapered into a semispherical nose. The wing combined a high coefficient of lift with low drag. The aircraft’s structure combined unprecedented strength with flexibility, allowing a wing loading of 69lb per square feet – double that of the B‑17 and 50 percent greater than the B-24.

A monocoque structure, it achieved maximum strength at minimum weight through a combination of internal framing and thick skin. Longitudinal frames on the fuselage were eight inches apart. The skin was the thickest used to date on an aircraft – on both the fuselage and inner wing it was 0.1875in. thick. Wing spars were similarly robust, with their flanges weighing 255lb. The wings had a 4.5-degree dihedral and a seven-degree leading edge sweep-back, which improved stability. The fuselage consisted of five major assemblies – the forward pressurized compartment, the bomb-bays, the wing gap closure, the aft section (with pressurized and unpressurized compartments), and the tail gunner’s compartment. The midsection (with the bomb-bays and wing gap closure) and the rear half of the aft section were unpressurized. The forward and aft compartments and the tail gunner’s compartment were all pressurized. The wing had three major assemblies – the inboard wing, which ran from the starboard to port outer engine nacelle, and two outer wings, port and starboard. These were made by subcontractors at various major B‑29 assembly plants.

The forward pressurized compartment accommodated both pilots and the bombardier, navigator, radio operator, and flight engineer. The aft pressurized compartment was where three gunners were stationed, and it also contained the toilet and bunks. The rear compartment housed the tail gunner. To allow travel between the forward and aft pressurized compartments, a pressurized tunnel ran the length of the bomb-bay. Movement between the tail gunner’s compartment and the aft pressurized compartment could only be undertaken when the bomber was depressurized. Cabins were equipped with sound-dampening and temperature-insulating fiberglass blankets.

Superchargers maintained a cabin pressure of 8,000ft up to 30,000ft in these compartments, and an air conditioning system heated or chilled the air to 70°F. The cabin was depressurized 30 minutes prior to combat, cooling the cabin temperature. When depressurization began, the crew donned oxygen masks to breathe and plug-in flight suit heaters to stay warm. Smoking was allowed when the aircraft was pressurized.

ENGINES

The B‑29 was powered by four 18-cylinder Wright R‑3350 Duplex-Cyclone radial engines. Each powerplant had two rows of nine cylinders set radially around a central axis. Each cylinder had a 6.125in. diameter and a 6.3125in. stroke. Total engine displacement was 3,347.9 cubic inches. The R‑3350 had a pushrod drivetrain and was equipped with a two-speed single-stage turbo-supercharger. The dry weight of the engine was 2,670lb. It burned either 100 or 130 octane aviation gasoline. Generating 2,200hp in the version fitted to the B‑29 during World War II, the R‑3350 was one the conflict’s most powerful aircraft engines. Its development had started in January 1936, in part as a response to the competing Pratt & Whitney R-2800 Double Wasp. A prototype of the R‑3350 was first run in May 1937, but progress was slow over the next two years. Wright delivered only seven engines to the USAAC for ground-running between 1936–39.

Development accelerated after the USAAC released the RFP for a “super bomber” in December 1939. The performance required by the USAAC dictated the creation of an engine capable of producing more than 2,000hp. The only powerplant meeting this requirement was the R‑3350, which had not yet flown – the first examples to take to the air were pre-production engines installed in the XB-19 in June 1941. All four bomber designs submitted in response to the RFP specified the R‑3350 for their engines.

Wright received its first production order for the R‑3350 in May 1941, but design changes, low manufacturing priority, and factory limitations meant the first examples were not delivered until year-end. Following the attack on Pearl Harbor, and with the USAAF’s growing interest in the “super bomber,” Wright received better financial support for engine construction.

ARMAMENT The B‑29 was one of the most heavily armed bombers of World War II, originally being equipped with 12 0.50‑cal. AN/M2 Browning machine guns and one 20mm M2 cannon (the US Army and the US Navy used identical weapons, especially for aircraft, with commonality being indicated by the designation “AN” for Army–Navy). Ten of the 0.50‑cal. machine guns were in unmanned, remotely controlled General Electric power turrets. Two 0.50‑cal. machine guns and the sole 20mm cannon were in a manned position in the tail. Both weapons were air-cooled, and mechanically reliable.

The M2 was a US licensed version of the Hispano- Suiza HS.404 Mk II 20mm cannon. With a muzzle velocity of 2,800ft per second, it fired a 4.6oz projectile (armor-piercing, highexplosive or incendiary), with a rate of fire of 700 to 750 rounds per minute. The high-explosive round charge was between 0.21–0.39oz. The AN/M2 0.50‑cal. fired a 52-gram bullet with a muzzle velocity of 2,910ft per second. Capable of expending 750–850 rounds per minute, the AN/M2 was particularly effective against lightly armored Japanese fighters – its rounds could penetrate one inch of armor.

The B‑29 had front upper, aft upper, front lower and aft lower turrets. The front upper turret had four 0.50‑cal. machine guns, while the rest had two each. They were operated by three gunners (upper, right and left) and the bombardier. The gunners occupied sighting stations located at Plexiglas blisters in the middle of the fuselage. The turrets were operated using the General Electric Central Station Fire Control System. It allowed any gunner to operate all the turrets, or just their primary station. The bombardier had primary control of both forward turrets. The upper gunner had primary control of the aft turret and secondary control of the front turret. The waist gunners split primary control of the lower aft turret and secondary control of the lower front turret. The tail gunner fired the tail turret guns.

ELECTRONICS

The B‑29 carried the most sophisticated electronic suite of any aircraft in World War II. Although designed before AI radar and electronic countermeasures (ECM) existed, the aircraft had sufficient volume and carrying capacity that these could easily be added as retrofitted equipment. Much of the bomber’s electronics were installed during the manufacturing process, rather than as field modifications. This improved reliability.

The B‑29 carried an SRC-269 radio compass, AN/APQ-4 navigation system, SCR-695 radio identification system, RC-43A marker beacon receiver, SRC-570 landing guidance system, AN/ARN-7 radar compass, AN/APQ-13 or AN/APQ-7 navigation and bombing radar, and an SCR-729 or AN/APN-2 radar interrogation system. It could also be equipped with AN/APQ-9 and AN/APR-4 radar jammers and the AN/APT-1 airborne radar warning system.

The navigation equipment permitted the B‑29 to accurately reach its target and then return safely home despite flying nearly 1,500 miles each way. The SCR-269 radio compass gave greater accuracy than the standard magnetic compass. The AN/APQ-4 used broadcast signals to fix the position of an aircraft – postwar, it became the Long-Range Navigation (LORAN) system. The AN/APQ-4 worked up to 1,500 miles from the ground broadcast station (roughly the distance from Saipan to Tokyo), with an accuracy of tens of miles. The AN/ARN-7 was a radar that could be used for homing. The AN/APQ-13 was an advanced navigation and bombing radar. An improvement of the H2X (AN/APS-15) radar, it increased bombing accuracy enough to justify redesigning the B‑29 to allow it to become standard equipment. The radar allowed blind bombing of city-sized targets. The AN/APQ-7 improved accuracy still further, permitting precise radar bombing of large facilities like oil refineries and shipyards.

Extract from B-29 Superfortress vs Japanese Nightfighter.