The Boeing B-17C was produced for the Royal Air Force as the "Fortress I." Due to the war, the Fortress was greatly modified internally, and added armament incorporated. A total of 38 were built, 20 of which went to the RAF. The first stratosphere bombing raids of the war were done with these airplanes when RAF crews plastered German ports. This raid marked the actual beginning of sub-stratosphere warfare, and forced redesign of exiting fighters to escort the bombers, or in the case of the Nazis, to cope with them.