were Boeing production B-17Cs (company designation Model 299T). The aircraft's single .30 caliber nose-mounted machine guns were replaced with 0.5 inch Brownings. Following their delivery, the 20 B-17C bombers were placed immediately into frontline service and designated RAF Fortress Mk I. They performed unremarkably while in British service. By September 1941, three months after the Army Air Corps became the Army Air Forces, 39 sorties had made up 22 missions. Nearly half of those were aborted due to mechanical and electrical problems. Eight of the 20 aircraft were destroyed by September, half to various accidents. Their machine guns tended to freeze-up at high altitudes and were generally unable to effectively protect the Fortresses from German fighter attack. Their success as bombers were also limited, largely because they were unable to strike targets from the high altitudes at which the RAF flew its daylight bombing missions. The first of the B-17C series flew in July 1940, with a total of 38 being built. The 18 remaining in Army Air Forces service, following the 20 transferred to the RAF, were upgraded to Boeing's new B-17D configuration. However, one of these bombers, B-17C 40-2047, crashed while being ferried from Salt Lake City, Utah, to Mather Army Air Base, California, on November 2, 1941. B-17D Though changes in the design made the Army Air Force decide that the B-17D was worthy of a new sub designation, the B-17C and B-17D were very similar. In fact, both were given the same sub designation (299H) by Boeing. Minor changes were made, both internally and externally. Outside, the engines received a set of adjustable cowl flaps for improved cooling, and the externally-mounted bomb racks were removed. On the interior, the electrical system was revised, and another crew position was added, bringing the total number to ten. In the aft-dorsal radio compartment was a new overhead twin-.50s machine gun mount; in the central-aft section's ventral "bathtub" gun position, twin .50s were also added, as was additional armor plating. Nose gun ball sockets were added to the side windows for the first time, in a longitudinally staggered layout (the starboard ball socket was further forward than the port-side ball socket). The number of machine guns aboard brought the total armament to seven: one portable nose .30 in (7.62 mm) and six .50 in (12.7 mm). The B-17D also featured more extensive armored plate protection. A total of 42 "D"