well no matter what you find about takeoff rates, there is one thing about the B17... it was definately not the best bomber in the theatre, but because of both the popularity it recieved and the numbers in which they were built, it is the most famous.
About 12,000 B17s were produced compared to about 7300 Lancasters.
Also, we have some very stunning videos and photos of B17s flying dangerous missions into Germany with flak bursting all over, valiantly flying straight and level to bring bombs to the doorstep of the Axis.
But the only videos we have of the British night bombing are peaceful little shots of them taking off at dusk, or from the belly of a Lancaster as the helpless German town is ridden with incindiary bombs while the Lancasters float in the dark with much less risk than the american bombers.
The Lancaster probably was the best, but the thing is that the British still chose the B24s to do any day bombing or patrol work they had. So no one really knows. A Lancaster pilot will tell you his was the best, and the 17 pilot says his was the best.
Its a little BS propaganda and a little truth. Take everything with the b17 and Avro 683 with a little grain of salt.