first flight on 19 May 1943. While it showed a slightly higher top speed, after just a few flights it had to be grounded due to a problem with engine manifold joints leaking exhaust. Following the fixing of this problem, testing continued until the ninth flight on 16 June 1943. During this flight, the starboard (third right) inboard engine caught fire, and the crew was forced to bail out. The XB-38 was destroyed and the project was cancelled. The gains in modification were minimal and would have been disruptive to the existing Flying Fortress production. Allison engines were also considered to be more badly needed for constructing fighter aircraft. YB-40 Prior to the introduction of the P-51 Mustang, a B-17 "gunship" escort variant called the YB-40 was introduced. This aircraft differed from the standard B-17 in that a second dorsal turret was installed atop the radio operator's position between the forward dorsal turret and the waist guns, where only an upward firing single or double Browning M2 had been mounted; and a single 0.50 in (12.7 mm) machine gun at each waist station was replaced by a pair of 0.50 in (12.7 mm) guns, of basically the same twin-mount design used for the tail guns. In addition, the bombardier’s equipment was replaced with twin 0.50 in (12.7 mm) machine guns in a remotely operated "chin" turret directly under the bombardier's position, augmenting the existing "cheek" machine guns; and the bomb bay was converted to a .50 caliber magazine. The YB-40 would provide a heavily armed gunship escort capable of accompanying the bombers all the way to a target and back. The aircraft was deemed a failure, however, because it could not keep up with standard B-17s once they had dropped their heavy bomb loads. It was withdrawn from service after just fourteen missions. (Twenty-six were built: one XB-40 prototype, 21 YB-40 pre-production aircraft, four TB-40 training aircraft.) C-108 Flying Fortress Four B-17s were converted to serve as cargo carriers and V.I.P. transports under the designation C-108 Flying Fortress. (Many more served in the same roles under the designations CB-17 and VB-17, respectively.) The first of them, designated XC-108, was a B-17E partially stripped of military equipment and outfitted with various living accommodations. It served as a V.I.P. transport for General Douglas MacArthur. A similar conversion was made on a B-17F, which was redesignated YC-108. The third plane, designated XC-108A, was