had ventral "bathtub turrets" (see the "C/D" section below) installed, replacing their lower, teardrop-shaped gun blisters. Crew locations were rearranged, and the original pneumatic brake system was replaced with more efficient hydraulic brakes. In October 1942 all in-service B-17B aircraft were redesignated RB-17B, the "R" indicating "Restricted". These aircraft were now used only for training, transport, messenger, and liaison duties. The "R" prefix became a designation for combat obsolescence. Many of these RB-17B aircraft, along with at least one still-airworthy YB-17, were stationed at Sebring Airfield, where the exterior scenes were filmed for the Warner Bros. war drama Air Force (1943), directed by Howard Hawks, and starring (among others) John Garfield, Arthur Kennedy, Gig Young, and Harry Carey. The film's real star, however, was an RB-17B (United States Army serial number 38-584), carrying on its upper rudder the "security-conscious" false serial number "05564". It passed as a later model B-17D Flying Fortress, having had its machine gun blisters replaced and a lower "bathtub" ventral gun turret installed. Many of these aircraft can still be seen in both ground and aerial scenes during the film. The "B" series Flying Fortress made its maiden flight on 27 June 1939. 39 were built in a single production run, but Army Air Corps serial numbers were scattered over several batches. This was because of limited government funding: The Army Air Corps could only afford to purchase a few B-17Bs at a time. B-17C The B-17C was a B-17B with a number of improvements, including more powerful R-1820-65 engines. To boost crew safety, the waist-mounted machine gun blisters were replaced with teardrop-shaped, slide-out Perspex window panels flush with the fuselage, and the ventral blister was replaced by a lower metal housing dubbed a "bathtub turret", similar in appearance and general location on the lower fuselage, to the Bola ventral gondola being used on Nazi Germany's He 111P medium bomber. The most important additions made to the "C" series were self-sealing fuel tanks and defensive armor plate added to vital areas. With the passage of the Lend-lease Act in 1941, the Royal Air Force requested B-17s. At that time, the US Army Air Corps was suffering from shortages of the B-17, but hesitantly agreed to provide 20 examples to the RAF. Though the Air Corps did not consider the B-17 ready for offensive combat, the aircraft was still desperately needed in Britain. The 20 ferried bombers