Daylight bombing from 25,000 feet in a box of guns — and the proof that bombers alone could not win.
1943·472 km/h at 7,600 m·1,800–3,600 kg typical·4× R-1820 turbocharged radials, 1,200 hp each·10,850 m·12,731
1· THE STORY
The fortress that needed friends
The Americans came to Europe believing that heavily armed bombers, flying high in tight formations by daylight, could fight their way to any target and hit it precisely. The B-17 was that belief made in aluminium: four engines with exhaust-driven turbochargers that held their power at 25,000 ft, thirteen heavy machine guns, and the Norden bombsight. In 1943 the belief broke over Schweinfurt, where sixty bombers were lost in one raid and seventy-seven in another. What saved the bombers was not more guns but the long-range fighter that arrived the following spring.
472 km/h at 7,600 mTop speed1,800–3,600 kg typicalBomb load
2· WHY IT MATTERED
The combat box
A 1943 survey found that most of the bombers lost had drifted out of formation. So formations became a stacked box of eighteen to thirty-six aircraft, arranged so that every gun could see every approach and a fighter attacking one bomber flew through the fire of several. It worked, up to a point, and it meant flying straight and level through the flak.
The Norden bombsight was a gyro-stabilised analogue computer: the bombardier fed it altitude, speed and wind, kept the crosshairs on the target, and for the last minutes it flew the aircraft through the autopilot and released the bombs itself. In training it was uncanny. In combat, through cloud, flak and formation drift, bombs landed hundreds of metres from where the sight said they would.
4· WHY IT MATTERED
Schweinfurt, and the lesson
On 14 October 1943, 291 B-17s attacked the ball-bearing works at Schweinfurt beyond the range of any escort. Seventy-seven did not come back. Daylight raids deep into Germany stopped until the P-51 could go with them; when it did, in February 1944, losses fell below seven per cent and the campaign resumed.
5· THE ENGINE
Turbochargers: the altitude engine America had
Every engine loses power as it climbs into thinner air. The B-17’s answer was a turbine in the exhaust stream, spinning at hundreds of degrees, driving a compressor that fed the engine sea-level air at 25,000 ft. The prototype without turbos managed 385 km/h and a ceiling of 8,500 m; the same airframe with them did 500 km/h and 11,600 m. General Electric’s turbocharger was a technology Britain and Germany largely lacked, and it is why American bombers flew so high.
4× Wright R-1820-97 Cyclone · 9-cylinder radial · 29.9 L · gear-driven supercharger plus a GE exhaust turbocharger · 1,200 hp each, held to 25,000 ft
Thirteen heavy machine guns firing 43-gram bullets at 800 rounds a minute add up to about 7.5 kg of metal a second — on paper. In practice no attacker could be reached by more than a few of them at a time, and a Fw 190 with four cannon firing head-on had the better of the exchange. The bombs were what it was for: two tonnes to a target 1,300 km away, more on shorter trips.
7· TRY IT
Turbo or not: the same 1,200 hp against altitude
The lab runs the Cyclone with and without its turbocharger. Without, its power follows the air density down to nothing; with, it holds 1,200 hp to 25,000 ft. The two prototypes’ real numbers — 385 km/h and 8,500 m against 500 km/h and 11,600 m — are on the chart to check against.
Readout
Weight of fire7.45 kg/s
Cannon share0 %
Seconds of fire30 s
Metal in this burst14.9 kg
Boeing B-17 Flying Fortress: the guns as carried on the main version; seconds of fire is the gun that runs dry first.