The Boeing B-29 Superfortress was the most expensive single weapons programme of the Second World War. It cost more than the Manhattan Project. Its development consumed three billion dollars of 1944 money — call it sixty billion today — and produced an aircraft that introduced more entirely new engineering concepts to one airframe than any combat aircraft before or since. Pressurised cabins. Remote-controlled gun turrets. A computerised fire-control system. Wings so long that they generated their own structural problems. And — most curiously of all — a 34-inch-wide padded tunnel running the length of the bomb bay so the crew could crawl from the front of the aircraft to the back without dying.

The tunnel was not a luxury. It was the only way to make a pressurised bomber that could also drop bombs. And in 1944, the engineers who designed it solved a problem that still seems improbable today.

The problem nobody had solved

Pressurised airline cabins were a 1930s invention. The Boeing 307 Stratoliner had flown in 1938. But every pressurised airliner had something a bomber did not: a continuous, sealed fuselage with no holes in it. The B-29 needed to fly at 30,000 feet to escape Japanese fighter cover and flak, which meant the crew had to be pressurised. The B-29 also needed bomb bays that opened in flight, which meant the centre of the aircraft could not be pressurised — you cannot open a hole in the side of a balloon and expect the balloon to stay inflated.

Boeing’s engineers split the difference. The forward crew compartment — pilot, co-pilot, bombardier, navigator, radio operator, flight engineer — was pressurised. The rear compartment — three gunners, the central fire control gunner, and a tail gunner — was pressurised. The bomb bays in between were not. To move between the two pressurised volumes during flight at 30,000 feet, the crew used a 34-inch-diameter padded tunnel that ran OVER the bomb bays and connected the two pressurised compartments through twin pressure-tight bulkheads.

Inside the tunnel

The tunnel itself was a tube of stretched aluminium skin reinforced by hoop frames, lined inside with quilted padding and small canvas straps that crewmen used to pull themselves through. At each end, an oval pressure door — locked, dogged, and gasketed like a submarine bulkhead — sealed the tube against the unpressurised bomb-bay air outside. To open the door without depressurising both compartments, a crewman first had to confirm the tunnel itself was pressurised. There was no shortcut.

Crewmen who had to make the trip during a long flight described it as awkward but tolerable. The trick was to remember that the tunnel ran above the bomb bay, which meant entering it meant climbing up and forward. Officially the journey took less than a minute. Unofficially, in an aircraft that was being shaken by flak, the minute felt rather longer.

Why nobody had built it before

The engineering challenge was not the concept of a tunnel. The challenge was that the tunnel had to maintain pressure differential — about 8 PSI between cabin pressure (set to roughly 8,000 ft equivalent) and outside ambient at 30,000 ft — while running through the structurally weakest section of the aircraft, the bomb bay roof. The aluminium skin of the tunnel had to be stiff enough to handle the pressure, light enough not to ruin the weight budget, and tight enough that thousands of rivets did not slowly bleed pressure out into the slipstream.

Boeing engineer Edmund T. Allen — who would later die in the second prototype on 18 February 1943 when an engine fire caused a catastrophic mid-air structural failure — wrote a remarkable memo in 1941 setting out the design philosophy. The tunnel had to “preserve the conditions of life” between the two pressurised volumes, but had to be “secondary in structural importance to the bomb bay.” It was, in other words, a comfort feature welded onto a wartime bomber by men who had no engineering precedent at all.

What it made possible

The tunnel was not just for moving. It was also a sleeping platform on long missions over the Pacific. The Tokyo raids of 1944–45 routinely lasted fifteen hours. Crew members who were off duty would crawl into the tunnel, brace themselves against the padding, and sleep. The tunnel had its own oxygen supply, its own intercom tap, and — on later production aircraft — its own small heater. It was, by accident, the first ever in-flight crew rest compartment in a military aircraft. The same idea would not return until the Boeing 747’s upper deck thirty years later.

When the B-29 dropped the atomic bombs on Hiroshima and Nagasaki, the crews who carried them out had crawled through that tunnel to swap positions during the long approach from Tinian. It was a piece of 1941 engineering that quietly outlasted the world it was built for.

Sources: Wikipedia; Pilotmall.com; nuclearcompanion.com; New England Air Museum archives; airplanes-online.com; period Boeing engineering memoranda.