The Boeing B-29 was the most advanced bomber of World War Two, and more expensive to design and build than the atomic bombs it dropped. It also helped influence the airliners we fly on today.

It was two years before Japan’s attack on Pearl Harbour dragged the United States into the war. But the US Army Air Corps was looking for a new bomber aircraft. What they were after was a "superbomber", capable of flying up to 2,000 miles (3,200km) at a time and at altitudes never achieved before.

The aircraft they got would go on to drop the atomic bombs on Hiroshima and Nagasaki and ultimately bring an end to World War Two. It would also pave the way for a civil aviation boom that led to the everyday air travel we have today.

This is the story of how an aircraft that cost more than the entire Manhattan project – the B-29 Superfortress – changed the world.

It was January 1940 when the US Army Air Corps (USAAC) approached five American aircraft companies with the request to build a bomber bigger than anything the world had ever seen. Although the US was yet to enter the Second World War, in Europe it was already raging: Nazi Germany and the Soviet Union had only months earlier invaded Poland and divided it between them. The US knew it was a matter of time before it might be dragged in.

The aircraft the USAAC wanted would have to fly further and higher than any aircraft that had yet been built. That turned out to be an enormous challenge, even for the world's biggest industrial nation.

Two of the companies approached, Douglas and Lockheed, soon abandoned work on their submissions due to the problem it posed. Boeing, however, had a head start, having begun work on a design as a private project a few years earlier.

Boeing's XB-29 design eventually won the USAAC's competition, but it would be another four years before the aircraft that became known as the B-29 Superfortress entered service.

It was the most expensive and complex industrial project US industry had ever undertaken and would not be surpassed until the space programmes in the 1950s and 1960s. And it pushed aviation technology almost to the limit.

The B-29 project became the most expensive of the entire war – costing nearly 50% more than the Manhattan Project that built the world's first atomic bombs. In today's money the aircraft, from design to completion, cost the equivalent of $55.6bn (£41.2bn).

The bombers that Boeing and other plane makers built for the first few years of World War Two tended to carry out their missions above 20,000ft (6km). The higher you fly, the longer you can fly, because the air is less dense, but it presented the crews inside the planes with many challenges.

"You have to be on oxygen the whole time," says Hattie Hearn, the curator of the American Air Museum in Duxford in the UK. "Within two minutes of not being on oxygen you probably lose consciousness, and that was a real common thing that air crew had to had to consider. They had to have electrically heated flying suits, all this other gear which is going to be impeding movement. Very cumbersome, if you had to escape, for example. Often you could get too hot, and then that would again impede how you operated, if you're sweating, and then suddenly that sweat freezes."

Conditions were bad enough at 20,000ft. At 30,000ft, the height the B-29 needed to fly, they would be even worse.

Boeing turned to a promising new concept: pressurisation. This meant the plane's cabin had the same air pressure and oxygen content you would find on the ground – which meant crew didn't have to fly with oxygen masks. The air would be pulled out of the engines, cooled, purified and then pumped into the crew compartments. It could also be reheated so the crew didn't have to wear bulky flight suits.

The technology had been under development since the 1920s, but the concept was still relatively experimental. The B-29 would become both the first bomber aircraft and the first mass-produced aircraft to be pressurised.

"Allowing a crew to operate in light flight suits, because the compartments are heated, that allows them to take these long missions in a much more hospitable way," says Jeremy Kinney, a curator at the Smithsonian National Air and Space Museum in Washington DC.

Pressurising the aircraft's entire hull would be both difficult and expensive. Instead, Boeing devised three separate pressurised compartments for the B-29's 11-person crew. The first compartment in the nose of the aircraft housed the pilot, co-pilot, bomb aimer, navigator, radio operator and flight engineer – the latter responsible for monitoring the airplane's four enormous, temperamental engines. Between the first and second crew compartments – and above the first of the plane's massive bomb bays – was a tunnel that the crew members could crawl through. The middle compartment, where the gunners and radar operator worked, even had a chemical toilet and some bunks to nap on.

"There were some risks involved," cautions Hearn. "If you suddenly had a loss of pressure and you're in the tunnel moving between the different cabins, you could be shot out pretty much as fast as a bullet, which is a bit of a worry."

The high speed of attacking planes in World War Two........

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