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.

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.

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.

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.