Eighty-five USAAF B-17Gs were transferred to the Royal Air Force as the Fortress III. The first 30 of these planes were built by Boeing, and the remainder were built by Lockheed-Vega. RAF serials were HB 761/790 for the Boeing-built models and HB791/793, 795, 796, 799/803, 805, 815/820, KH998, KH999, and KJ100/127, KL830/837 for the Vega-built models. However, HB794,797,798,804, 806/814 were diverted to the USAAF before they could be delivered to the RAF.
Three of these Fortress IIIs (HB786, HB791, and HB792) were operated by RAF Coastal Command. They all served with 220 Squadron in the Azores and were subsequently allocated to two met squadrons (HB786 to 521 Sqdn and HB791 and HB792 to 251 Sqdn). They usually carried twelve 400-pound S.C.I. bombs or sixteen 250-pound depth charges. They had the cheek-mounted machine guns removed, and radar was fitted in place of the ball turret.
In February 1944, the first Fortress IIIs were assigned to No. 214 Squadron of Bomber Command based at Sculthorpe. These operated with with No. 100 group on special electronics countermeasures missions to confuse and jam enemy radar. With No. 233 Squadron, the first Fortress III unit of Bomber Command, they took part in clandestine operations until the unit was disbanded in July of 1945. Fortress IIIs also took part in mass night attacks, being employed as decoys to confuse enemy night fighters and to drop "window".