From: Edgewater, MD
I'll throw two more factors into the pot - formation flying penalties and course changes.
All manufacturer aircraft range data (I work for one) assumes the plane is flying alone and straight. A formation of heavies on a bombing run is obviously doing neither.
The 'lead' in a formation may burn fuel at the theoretical rate, but no one trying to hold station on them will unless they're Gods come down to Earth (of course, many pilots feel they themselves are, but...). Minor variations in wind, turbulence, etc. force you to constantly adjust your indicated airspeed, which wastes fuel. Not a lot - but on a thousand-mile run it will be noticeable.
Worse, however, not everyone can take off (or land) at once. Since the formation needs to arrive over target at the same time the first birds off the ground burn fuel while everyone else gets off the ground and climbs. The formation then needs to form up - more fuel lost, especially if you're coming from multiple bases. The USAAF in the ETO actually theater-modified veteran bombers with eye-catching full body paint jobs and load of flares just for this purpose - they'd take off, server as a reference point for a formation taking station, then land without flying the mission itself.
Course changes were mentioned above re: Ploesti - the game model assumes your A/C fly straight from their base to the target, attack, and fly straight back. Doglegs, often large ones, were common in flight plans to avoid expected fighters, flak, or weather. Each turn, for that matter, burns more fuel as planes jockey to keep relative station.
Probably more useful to look for detailed data on actual missions flown (with what bombload, aux tanks, armament, etc.), paying particular attention to ops losses.
While I'd love to see B-17 range go up, I have to ask if there are out-and-back missions that were historically flown with 'standard' loads and configurations that can't be flown in game? Flying FARP missions being beyond the game engine, sadly, means they don't count, along with any missions like the Doolittle raid where the A/C on the flight were modified.
Those items are factored into the algorithm I am using to establish my Rng Normal and Rng Extended values. I'm using the flight manuals to establish the Baseline Ranges I will plug into my algorithm.
Here's a link to: Treespider's Grand Campaign of DBB
"It is not the critic who counts, .... The credit belongs to the man who is actually in the arena..." T. Roosevelt, Paris, 1910