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.
Boeing B-17 Flying Fortress, 6 of 10
Deep dive · The turbocharger· opened from Boeing B-17 Flying Fortress
Timelines · WW2 aircraft · go deeper · Boeing B-17 Flying Fortress
The turbocharger1938
A turbine in the exhaust, spinning in red-hot gas, that gave the engine sea-level air at 25,000 ft.
Exhaust turbine, compressor, intercooler: boost that costs no crankshaft power
A supercharger driven off the crankshaft takes power to make power. A turbocharger takes its drive from the exhaust instead: a small turbine sits in the hot gas leaving the cylinders and spins a compressor on the same shaft, and the energy that would have gone out of the pipe as heat and noise goes into pushing air in. The catch is the turbine, which lives in gas at 800 °C, and in 1938 only General Electric could make one that lasted.
02Why it matters
Same engine, 60 mph faster, two miles higher
The Y1B-17 without turbochargers did 239 mph and a ceiling of 27,800 ft. The Y1B-17A with them, on the same Cyclones, did 311 mph and 38,000 ft. The engines made no more power at sea level; they kept it as they climbed, and the aircraft climbed to where the flak and the fighters had trouble following.
03Why it matters
Why Britain went mechanical
Rolls-Royce looked at turbocharging the Merlin and rejected it: the ducting and the intercooler added weight and bulk a fighter could not spare, and the turbine alloys were not there. Their answer was the two-stage mechanical supercharger, which cost power but fitted. Both routes worked; the American one was the only one that suited a four-engined bomber.
04Why it matters
The waste gate
A turbo that always ran flat out would over-boost the engine at low altitude. A valve in the exhaust, the waste gate, bypasses gas around the turbine so that the boost stays at the set value from take-off to the height where the gate is fully shut — the critical altitude, 25,000 ft on the B-17.
05Try it
Turbo or not
The same 1,200 hp with and without its turbocharger, against altitude.
Readout
Power with turbo4,800 hp
Power without2,266 hp
Speed with turbo472 km/h
Speed without373 km/h
Calibrated to the two prototypes: Y1B-17 239 mph and a 27,800 ft ceiling; Y1B-17A 311 mph and 38,000 ft, on the same engines.