Boeing B-17 Flying Fortress

The Flying Fortress began as an answer to a 1934 US Army Air Corps requirement for a multi-engined bomber to defend the American coastline. Boeing gambled on four engines where the specification implied two, and the Model 299 first flew on 28 July 1935. A journalist looking at the gun positions bristling from its fuselage called it a “flying fortress”, and Boeing kept the name.

The prototype crashed on 30 October 1935 because the crew took off with the control locks still engaged, killing two men, and the Army bought the rival Douglas design instead. Boeing survived on a small service-test order — and the accident produced something that outlasted the aeroplane: the modern pre-flight checklist, introduced because an aircraft had become too complex to be flown from memory.

The B-17 is inseparable from a theory. American air power doctrine in the late 1930s held that a formation of heavily armed bombers could fly to a target in daylight, bomb it precisely and defend itself on the way — no escort required, because the massed fire of their own guns would make interception too expensive. The British, having tried daylight raids in 1939 and been shot out of the sky, had already abandoned the idea and bombed at night.

The precision half of the theory rested on the Norden bombsight, a mechanical analogue computer classified at the highest level and sold to the American public on the promise of putting bombs in a pickle barrel from 20,000 feet. On a test range in clear air the claim was not absurd: the Norden demonstrated a circular error probable of about 75 feet. Over Germany in 1943, through cloud, flak and evasive manoeuvre, the average was about 1,200 feet — sixteen times worse. Post-war audits of how many bombs actually fell within 1,000 feet of the aiming point range from 5% in the harshest studies to 31.8% in the most generous, and the spread itself is the honest answer: nobody was putting bombs in a barrel. The defensive half rested on geometry. Bombers flew in the combat box, a stepped formation designed so that every approach a fighter could take was covered by the guns of several aircraft at once — which meant hours of exhausting close formation flying, and bombing patterns dictated by the formation rather than by the bombsight. The doctrine, and the aircraft built to execute it, committed the Eighth Air Force to flying into Germany in broad daylight.

The clearest test of the idea came not over a German factory but over open water. On the morning of 4 June 1942, shortly after eight o’clock, fifteen Army B-17s flying from Midway attacked the Japanese carrier force from 20,000 feet — precisely the altitude and the method the aircraft had been designed and the crews trained for.

They scored no hits at all. A warship under way can see the bombs leave the bay and has minutes to turn; the bombsight that could put a stick of bombs into a factory yard could not solve for a target that moved while the bombs were falling. The four Japanese carriers were destroyed later the same day by dive bombers attacking from a few thousand feet, and the Army crews returned convinced they had hit several ships. Midway did not change American doctrine in Europe, where the targets stood still — but it settled, in one morning, what high-altitude bombing could and could not do to a fleet.

The definitive B-17G carried thirteen .50 calibre Browning machine guns — in the tail, the waist, the radio room, the ball turret slung beneath the belly, the top turret, and a chin turret added specifically because German fighters had learned to attack head-on, where the early Fortresses were weakest. Four Wright Cyclone radials took it to about 10,700 m, above much of the flak.

What it did not carry was a large bomb load: typically around 2,000–2,700 kg on a deep mission into Germany, against roughly 6,350 kg for an Avro Lancaster on a normal night sortie. Weight went into guns, armour and altitude instead. Roughly 12,731 were built. The trade was deliberate — the B-17 was designed to survive the trip, not to maximise tonnage — and it earned a reputation for coming home in conditions that should have destroyed it.

The theory was tested to destruction over the German ball-bearing industry. On 17 August 1943 the Eighth Air Force sent 376 bombers against Schweinfurt and the Messerschmitt works at Regensburg, at the limit of fighter escort range and well beyond it for most of the route. Sixty did not come back — sixteen per cent of the force in a single day, and one crew in six.

They went back on 14 October 1943, a day the Eighth still calls Black Thursday. Of 291 B-17s dispatched, 60 were shot down and 77 were lost or written off — a loss rate above 26%, with some 600 men killed, wounded or missing. At that rate a crew could not statistically complete a tour. Deep penetration raids beyond escort range were suspended, and the self-defending bomber was finished as a doctrine: it had been disproved by its own casualty lists.

There is one more figure from 14 October 1943, and it explains how the doctrine survived as long as it did. The bomber crews came home claiming 186 German fighters destroyed, 27 probable and 89 damaged. German records for the day show 31 fighters destroyed, 12 written off and 34 damaged — an overclaim of roughly six to one. The gunners were not lying: in a formation under attack, a dozen men fired at the same fighter as it flashed through, and each of them watched it go down. But it meant the Eighth was measuring the performance of its own defensive theory with an instrument that read six times high.

What made daylight bombing possible again was not a better bomber but a fighter that could go the whole way. From early 1944 the P-51 Mustang, fitted with drop tanks, could escort the formations to Berlin and back, and the arithmetic inverted: the bombers became bait, the escorts hunted the interceptors sent up to meet them, and the Luftwaffe’s day-fighter force began losing pilots faster than it could train them.

The change was immediate and measurable. In Big Week, from 20 to 25 February 1944, the Eighth and Fifteenth Air Forces threw thousands of escorted sorties against the German aircraft industry and accepted the air battle that followed — precisely the battle they had been unable to survive five months earlier. From that point the B-17 did what it had been built to do, in the campaign that flattened German synthetic fuel production and finally broke the Luftwaffe. The lesson is worth stating precisely: the aircraft did not fail at Schweinfurt and succeed in 1944. It was the same aeroplane both times, flown by the same kind of crews. What changed was that somebody was flying alongside it.

A tour was 25 missions — later raised to 30 and then 35 as the odds improved — and for much of 1943 the statistical chance of completing one was poor. That is why the crew of the Memphis Belle became famous simply for finishing: reaching 25 was an achievement worth a publicity tour of the United States.

The Eighth Air Force ended the war having lost roughly 26,000 men killed, a toll out of all proportion to its size. Its ten-man crews flew in unpressurised aluminium at −40 °C, on oxygen, for eight to ten hours at a stretch, with frostbite a routine injury and no way to treat a wounded man properly until landing. The ball turret gunner spent the mission curled in a plexiglass sphere slung below the aircraft, a position he could not enter or leave in flight while wearing a parachute.

The B-17’s reputation rests on toughness, and it is deserved: photographs of Fortresses landing with tails half severed, engines gone and whole sections of fuselage missing are not exceptional cases dressed up, they are a genuine characteristic of the airframe. Crews trusted it, and that trust was based on evidence.

One correction belongs here, because the aircraft’s fame has quietly rewritten the arithmetic. The Flying Fortress is remembered as the American bomber of the European war, and it was not the one that carried most of the load. In the Fifteenth Air Force from January 1944 the B-24 Liberator dropped 192,113 tons of bombs against the B-17’s 104,877, and across the whole war the B-24 dropped more tonnage than any other bomber flown by any air force. The B-17 was more photogenic, more survivable and far better known; the Liberator did more of the work.

Its historical importance, though, lies in what it disproved. The B-17 was the physical embodiment of the idea that bombers could look after themselves, it carried that idea into Germany in daylight, and the idea failed at a cost of thousands of men in the autumn of 1943. Only when the escort problem was solved by other aircraft did the strategy work — which makes the Flying Fortress both the most successful American bomber of the European war and the most expensive demonstration that a doctrine can be wrong.

• Donald L. Miller, Masters of the Air (Simon & Schuster, 2006) — the American bomber war.
• Roger A. Freeman, B-17 Flying Fortress and The Mighty Eighth (Arms & Armour / Cassell).
• Martin W. Bowman, B-17 Flying Fortress Units of the Eighth Air Force (Osprey).
• Edward Jablonski, Flying Fortress (Doubleday).
• Williamson Murray, Strategy for Defeat: The Luftwaffe 1933–1945 (Air University Press) — for the German loss records.