Boeing B-17 Flying Fortress (1943): the combat box

· 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 shot down in each of two raids and, after the second, seventeen more came home only to be scrapped. 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 speed (G)1,800 to 3,600 kg typicalBomb load

· 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.

GO DEEPERSupercharger vs altitude, the full demo →The turbocharger →

· WHY IT MATTERED

Precision from five miles up

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.

· WHY IT MATTERED

Schweinfurt, and the lesson

On 14 October 1943, 291 B-17s, mostly F models, attacked the ball-bearing works at Schweinfurt beyond the range of any escort. Sixty were shot down; another seventeen got back so badly damaged they were written off, and some 600 men were killed, wounded or missing. Daylight raids deep into Germany stopped until the P-51 could go with them; when it did, in Big Week in February 1944, losses over six days of raids fell to about seven per cent of sorties and the campaign resumed.

· 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 Y1B-17 service-test aircraft without turbos managed 385 km/h and a ceiling of 8,500 m; the Y1B-17A development aircraft with them did 500 km/h and 11,600 m on the same engines. Britain and Germany both experimented with exhaust turbochargers, but only the United States put them into production on this scale, 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

GO DEEPEROpen the full demo →

· WEAPONS AND SPECIALITY

Thirteen guns that never all fired at once

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: a theoretical muzzle rate with every gun firing at once. 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.

· 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 from about 1,500 m; with, it holds 1,200 hp to 7,600 m. Both curves are calibrated to the two development aircraft’s real numbers: 385 km/h and 8,500 m without, 500 km/h and 11,600 m with.

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.

· THE LAB

Power, guns and range: the same three models for all ten

Power against altitude from the type of supercharger, weight of fire against the other nine, and Breguet range with the armour and drop-tank trades. Simplified models, each scaled to this aircraft’s published figures, so the shapes are honest and the third decimal is not.

Boeing B-17 Flying Fortress: 7.45 kg/s, of which 0 % from cannon; 30 seconds of fire.

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 reference variant; seconds of fire is the gun that runs dry first. A bomber never brought every gun to bear on one attacker.