The service ceiling of the B-17 is 35K.
That cracks me up. I suspect you don't actually know what service ceiling is. And how difficult flying at altitudes above 30,000 feet actually is.
Maintaining altitude anywhere near service ceiling in the thirties or forties requires a high level of concentration by the pilots without an autopilot. There is no performance to spare. Maintaining a formation would be impossible. You are in coffin corner of the flight envelope, where the margin between critical mach and stall speed is very small.
Today's airplanes are air conditioned comfort. A B17 had the outside breeze blowing in. -70 degrees is darn common in the thirties. -70 with the wind blowing would make life as a waist gunner short and unpleasant. Right now, over Houston, the temperature at 34,000 is -54 F.
And we won't even discuss what running a radial engine at full power for several hours would do to it, especially trying to climb to high altitudes.
To imagine that the B17 regularly operated above 25,000 feet in large formations, heavily loaded is to be ignorant or just ignore the realities of such operations. All of the performance figures for best range and speed for the B17 are at 25,000 feet. That means performance dropped off above that altitude. And that is a brand new airplane with fresh motors. A few hundred hours of service and the airplane will not perform to book numbers anymore.
Most raids were in the high teens and low twenties for altitude.