Pressurised, turbocharged, remote-gunned: it burned Japan’s cities and dropped the two atomic bombs.
1944·575 km/h·up to 9,100 kg·4× R-3350 turbocharged radials, 2,200 hp each·5,230 km·3,970
1· THE STORY
The most expensive weapon of the war
The B-29 was the most expensive weapons programme of the war — three billion dollars, more than the atomic bomb it eventually carried — and the most advanced aircraft in it. Its crew sat in pressurised cabins at 9,000 m, its guns were aimed by computers from sighting stations, and its four engines each had two turbochargers. It was rushed into service with engines that caught fire, and then, in March 1945, its commander threw out the idea it was built for: the guns came out, it flew low at night, and it burned Tokyo. On 6 and 9 August 1945 two of them ended the war.
575 km/hTop speedup to 9,100 kgBomb load
2· WHY IT MATTERED
A cabin you could breathe in
At 9,000 m the outside pressure is a third of sea level. The B-29’s nose and rear compartments were pumped up to the pressure of 2,400 m and joined by a tunnel over the bomb bays, which stayed at outside pressure because bomb doors cannot hold a seal. The crew flew eight-hour missions without oxygen masks, in a fuselage that was, in engineering terms, a pressure vessel with wings.
Four remote turrets, each with two heavy machine guns, were aimed from sighting stations by gunners who never touched them; five analogue computers corrected for the aircraft’s speed, the bullet’s drop and the lead needed. One gunner could take over any turret. It was the first fire-control system of its kind in the air.
4· WHY IT MATTERED
LeMay throws the concept away
Precision bombing from 9,000 m over Japan failed: the jet stream blew the bombs off target and the engines burned. In March 1945 General LeMay stripped the guns and gunners out, loaded incendiaries, and sent the B-29s in at 1,500–2,700 m at night. Sixteen square miles of Tokyo burned on the first night. The aircraft designed to be untouchable at altitude did its work low and unarmed.
5· THE ENGINE
The Duplex-Cyclone: rushed to war, and it burned
Eighteen cylinders in two rows, 55 litres, 2,200 hp, and a cowling so tight that the rear row overheated. The R-3350 first ran in 1937 and flew in 1941, and it went to war before it was ready: it swallowed valves, its turbine blades failed in 800 °C exhaust, and its magnesium crankcase burned at 3,000 °C, through the wing spar in seconds. Take-off at 60 tonnes on a tropical runway was the most dangerous minute of a B-29 mission — and in spring 1944 the Air Force sent factory workers to the airfields in Kansas to make 150 aircraft fit to deploy.
4× Wright R-3350-23 · 18-cylinder two-row radial · 54.9 L · two-speed supercharger plus two GE turbochargers per engine · 2,200 hp each
Ten heavy machine guns under central fire control put about seven kilograms a second into a fighter that any two turrets could reach, and the tail carried a cannon as well until it was found to be useless. Its bomb bays held nine tonnes — five times a B-17’s usual load — and, on two aircraft modified under the Silverplate programme, one bomb each.
7· TRY IT
Pressurisation: a cabin at 2,400 m inside a sky at 9,000
The lab computes the cabin: outside pressure from the barometric formula at cruising height, the differential the fuselage skin has to hold, and the hoop stress in a tube of that radius and gauge. It shows why the bomb bays stayed at outside pressure and the crew crawled between the cabins through a tunnel.
Readout
Power, 4 engines8,800 hp
Per engine2,200 hp
True airspeed575 km/h
Service ceiling9,710 m
A simplified model calibrated to the published rating and top speed — the shape of the curve is the point, not the third decimal.
Boeing B-29 Superfortress, 10 of 10
Deep dive · A pressure vessel with wings· opened from Boeing B-29 Superfortress
Timelines · WW2 aircraft · go deeper · Boeing B-29 Superfortress
A pressure vessel with wings1944
Inside, the air of a mountain pass; outside, a third of it — and a tube of aluminium holding the difference.
9,000 m·2,400 m·~45 kPa·~1.4 m
01The idea
Cabin pressure, ambient pressure, and the hoop stress in the skin that keeps them apart
Air pressure falls with height roughly exponentially: at 9,000 m it is about a third of sea level, too little to stay conscious in for long. Oxygen masks were the usual answer and the B-17’s crews wore them for eight hours at a time. The B-29 pumped its cabins up instead, to the pressure of 2,400 m, which made the fuselage a pressure vessel: a tube whose skin is stretched by the pressure inside it like the wall of a balloon.
02Why it matters
Hoop stress
For a thin tube the stress in the skin around its circumference is the pressure difference times the radius divided by the skin thickness. A B-29 fuselage of about 1.4 m radius holding 45 kPa with a skin around a millimetre thick works out at tens of megapascals — well inside what aluminium can take, but every rivet line and every window is a place the stress concentrates.
03Why it matters
Why the bomb bays stayed outside
Bomb doors cannot hold a seal, so the two bomb bays between the crew compartments stayed at outside pressure. The nose and the rear cabin were connected by a tunnel over them, just wide enough to crawl through, so that the crew could move without depressurising anything.
04Why it matters
The mission that did not need it
When LeMay sent the B-29s in low at night, the pressurisation and the high-altitude engines were dead weight. The aircraft designed to be untouchable at 9,000 m did its most destructive work at 2,000, and the technology waited for the airliners.
05Try it
The cabin, live
Choose the cruising height and the cabin altitude; the skin stress follows.
Readout
Pressure difference44.9 kPa · 6.5 psi
Hoop stress in the skin65 MPa
Margin to yield× 4.3
Radius 1.45 m, skin 1 mm, alclad yield about 280 MPa; the real structure shares the load with frames and stringers. The bomb bays stayed at outside pressure — a pressurised cabin with doors that open at 9,000 m is not a cabin — so the crew moved between the two pressurised sections through a tunnel over them.