Douglas Aircraft was charged with making the modifications for the operational aircraft and, beyond the mentioned gun installations, other arrangements were trialed during this period - some involving automatic cannons of various calibers - though none made it to service.
The product was already on order back in October 1942 when a prototype "XB-40" made a first-flight on November 10th of that year. The USAAF required thirteen converted bombers through an initial batch and a further twelve were added in January of 1943. First production-quality aircraft emerged from Douglas that March and the series carried the developmental designation of "YB-40" for the interim, pending the outcome of their in-service performance.
The aircraft were delivered to England in May of 1943 (one was lost en route, crash-landing in Scotland) and these flew operational-level missions for the USAAF from that point until July of that year. Results were not wholly impressive as the fleet claimed only five enemy fighters from the forty-eight missions flown. The added weight of the armor plate and installed armament meant that these defenders could not keep pace with the main bomber force once the bombers had dropped their war loads. The test program was ended with the last mission flown on July 29th, 1943, this an attack involving a pair of XB-40s on the Kiel submarine pen.
Once their operational usefulness had concluded, the stock was sent back stateside and took part in crew training as the "TB-40". All were scrapped before the end of the war in 1945. The XB-40 was not a total loss as it introduced several key features of future B-17 generations - namely the Bendix chin turret (which proved vital in defending against oncoming attacks from the front) and the staggered beam gun positions. Work on the XB-40 also resulted in upgrades to the tail gunner's position in the way of improved vision for better tracking and engagement of fast-moving targets.
As completed, the YB-40 could boast a maximum speed of 292 mph with a cruise speed of 195 mph. Range was out to 2,260 miles with a service ceiling of 29,200 feet. Power was from 4 x Wright R-1820-65 turbosupercharged air-cooled radial piston engines developing 1,200 horsepower each.
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