American planners believed a tight formation of heavily armed bombers could fight its way to any target in daylight without escort — the “self-defending bomber.” Reality was harsher. Once B-17s flew beyond the range of their short-legged escorts, German fighters could attack almost at will. The two 1943 raids on the ball-bearing works at Schweinfurt exposed the flaw: the second, on 14 October 1943, cost 60 bombers shot down out of 291 dispatched, with dozens more written off — loss rates that no force could absorb for long. Unescorted deep daylight raids were effectively suspended.

The answer was not a better bomber but a better escort. Fitted with drop tanks, the P-51 Mustang could accompany the bombers to Berlin and back from early 1944, and in the “Big Week” battles of February 1944 the escorts began to break the Luftwaffe fighter force in the air. The B-17 remained the tool; the Mustang made the daylight campaign survivable.

The definitive B-17G carried thirteen 0.50-calibre M2 Browning machine guns — in the nose “chin” turret added to counter head-on attacks, in dorsal and ball turrets, at the waist, in the tail and cheek positions. The idea was collective defence: bombers flew in a stacked, overlapping combat box so that an attacking fighter faced fire from many aircraft at once. It helped, but it was never enough on its own against determined, cannon-armed German fighters attacking from head-on — which is why long-range escort mattered so much.

Each Wright R-1820-97 Cyclone produced around 1,200 horsepower, but its real trick was the General Electric exhaust-driven turbo-supercharger that maintained power in the thin air of high altitude. That let the Fortress operate far higher than most contemporaries. The cost was borne by the crew: cabins were unpressurised, temperatures could fall below −40 °C, and men worked on bottled oxygen in heated suits that frequently failed — frostbite injuries sometimes outnumbered wounds from enemy fire.

A wartime B-17 was, by modern standards, astonishingly cheap: mass production drove the flyaway price down to roughly $204,000–$238,000 across the war years — figures often quoted from US government wartime accounting, so treat them as order-of-magnitude rather than precise. There is no meaningful “cost per flight hour” for the type in its service life; it was a wartime state product built by the thousand.

Today the relevant number is the cost of keeping one of the handful of airworthy survivors flying. Operators of warbirds such as Sally B and (formerly) the Collings Foundation fleet have cited running costs of the order of several thousand US dollars per flight hour once fuel, insurance, maintenance and overhaul are counted — which is why B-17 flights are rare charity-supported events, not a commercial ride you can simply book.