A single Y1B-17 airframe (without military serial number) had been ordered as a non-flying article for use in static testing. It was to be delivered without military installations and tested to destruction to see how much punishment the airframe could take before it disintegrated.
However, the fact that Y1B-17 36-151 had survived an inadvertent spin led the Air Corps to conclude that the basic aircraft structure was exceptionally robust, so much so, in fact, that the static test was deemed unnecessary. On May 12, 1937, the Army decided to convert the static test Y1B-17 into a special flight test aircraft under the designation Y1B-17A. The plane was to be fitted with Moss/General Electric turbosuperchargers for its Wright GR-1820-51 (G5) Cyclone engines.
The Y1B-17A was designated Model 299F by the company, and the USAAC serial number 37-369 was assigned to the aircraft. The Y1B-17A flew for the first time on April 29, 1938. The turbosuperchargers were fed by a set of intercooler air intakes installed in the wing leading edges. Initially, the turbosuperchargers were mounted flat on top of the nacelles, but this arrangement proved to be unworkable. They were moved to the bottom of the nacelles, and the reworked aircraft flew again on November 20, 1938. This time, everything worked well. The turbosupercharged engines delivered 800 hp at 25,000 feet, whereas the R-1820-39s of the Y1B-17 could only deliver 775 hp at 14,000 feet. The increased high altitude power raised the operational ceiling of the aircraft to well above 30,000 feet and the maximum speed to 311 mph (as compared to 256 mph for the Y1B-17). The turbosuperchargers worked so well that they became standard equipment on the B-17B and on all future B-17 models.
The Y1B-17A was delivered to the Army on January 31, 1939. After completion of the service test period, the aircraft was redesignated B-17A.
Serial of Y1B-17A/B-17A
37-369 Boeing Y1B-17A Fortress -- later redesignated B-17A
Specification of Boeing Y1B-17A
Four supercharged Wright R-1820-51 radial engines rated at 800 hp at 25,000 feet, and 1000 hp for takeoff. Performance: Maximum speed 271 mph at sea level, 295 mph at 25,000 feet. Cruising speed 230 mph. Service ceiling 38,000 feet. Range 2400 miles with 4000 pounds of bombs, 3600 miles maximum range. Dimensions: Wingspan 103 feet 9 3/8 inches, length 68 feet 9 inches, height 14 feet 11 15/16 inches, wing area 1420 square feet. Weights: 26,520 pounds empty, 37,000 pounds gross, 45,650 pounds maximum. Armament: Armed with five 0.30-inch machine guns, with one gun in each of nose, dorsal, ventral, and two waist positions. A maximum of eight 600 pound bombs could be carried in an internal bomb bay.