The B-17 Flying Fortress first saw combat with the RAF, in the summer of 1941. The aircraft was offered to the RAF early in 1941, during the production run of the B-17C. Twenty of the thirty eight B-17Cs produced were delivered to the RAF in the spring and early summer of 1941, where they received the designation Fortress I. Forty RAF aircrew were trained at McChord Field, near Tacoma, Washington, from January-April 1941, before the aircraft was issued to No. 90 Squadron.
The B-17’s first combat mission was carried out on 8 July 1941 and was a high altitude daylight raid on the German naval base at Wilhelmshaven. By 12 September the squadron had carried out 22 raids, involving 39 Fortress sorties. Of those 18 had been aborted, two had bombed secondary targets and nineteen had bombed their primary targets. Only two 1,100lb bombs were recorded as hitting their target. In that period two aircraft were shot down and two more crashed on landing after being badly damaged. In all eight of the twenty aircraft were lost in two months and the Fortress was withdrawn from operations over Europe.
The performance of the Fortress confirmed the RAF in its belief that no daylight bomber could operate safely against the German air defences. The Army Air Force pointed out that the RAF was using the aircraft above its designed operating height and was badly overloaded, reducing its performance. The high altitude caused some of the guns to freeze up. The Americans also pointed out that the RAF were operating the Fortress in tiny groups, sacrificing the perceived benefits of mutual defence. The RAF experience did prove that the B-17 could not fly high enough to avoid the German fighters – the Bf 109E and Bf 109F could both intercept the Fortress at 32,000ft.
The Flying Fortress remaining in RAF service in limited numbers throughout the war. The most important user of the aircraft was Coastal Command. No. 220 squadron took over the Fortress Is of No. 90 Squadron, operating them in the Far East for two months, before receiving the Fortress II, which it used for anti-submarine patrols over the Atlantic. During 1942 and 1943 three squadrons used the Fortress for the same duty, one operating in the Far East. The two home based squadrons retained their Fortresses until 1944 and 1945 respectively. By 1945 the Fortress was being used by two Meteorological squadrons, one air-sea rescue squadron and one anti-submarine squadron.
The Fortress was also used by No. 100 Group on anti-radar and bomber support missions. No.214 Squadron received the type in January 1944, and No.223 in April 1945.
In RAF service the B-17C was designated as the Fortress I, the B-17F as the Fortress II and the B-17G as the Fortress III. The RAF received 20 Fortress Is, 19 Fortress IIs and 85 Fortress IIIs.