[Doolittle81]

I was wondering how much Flak noise could actually have been heard above the roar of the engines....So, a quick google got this personal account in a letter written at the time by a B17 crewmember/combat Photographer:

...Then the bomb bay doors swing open and we are on the bomb run. Flak bursting all around and our ship speeding in a bumpy fashion which is caused by the flak bursting entirely too close for comfort but still no damage to the ship...
...
At that second the ship jumped straight up several feet it seemed, and a sound like rocks on a tin roof filled my ears, even above the roar of the engines,, which at this point were turning over as fast as they possibly could. I felt my camera jump and saw fire fly from the front of it. I took the camera from my eye and could see light beaming through several places in the plane where flak had come through.
...
...

Both engines still purring, a very pleasing sound to anyone's ears, with the only other sound the shells going off at both sides of us. The sound of flak still leaving scars on the plane was to be heard constantly. At this point it was too late to worry, but not too late to think of a way out if the pilot gives the word, and hoping he maneuvers her out of the guns' range. This continued for minutes, it seems, but really only seconds.