In mid-1942, 45 B-17Es were turned over to the RAF as Fortress IIA. RAF serials were FK184/213, FL449/460, and FL462/464. The were called Fortress IIA because they were existing types transferred from the USAAF, whereas the Fortress IIs were allocated from new construction.
Probably because of its unhappy experience with the Fortress I, the RAF did not attempt to use their Fortress IIAs in the daylight high-altitude bombing role. Instead, they were turned over to Coastal Command for use in antisubmarine patrol work in the war in the Atlantic. The Fortress IIA entered service with No 220 Squadron on July 24, 1942. It is often reported that No 59 Squadron based at Thorney Island was the first to operate the Fortress IIA, but No 59 Squadron did not start operating the type until January 23, 1943. The third Coastal Command squadron to operate the Fortress IIA was No 206, which flew its first operational mission on September 19, 1942. Fortress IIAs also served with 251, 519 and 521 Squadrons
One Coastal Command Fortress IIA (FK185) was fitted with an experimental Bristol B.16 nose turret in place of the normal transparent fairing. This turret housed a 40-mm Vickers "S" gun which was remotely-controlled from a position just underneath the turret. The gun had a traverse of 30 degrees in azimuth and 40 degrees in elevation. It was intended for use against surfaced submarines. The modified IIA FK185 underwent firing trials but did not go into service. It was converted back to standard configuration and served with 251 Squadron
A few Fortress IIAs were issued to RAF Bomber Command, although I am unaware of their service history.