Boeing XB-15 model
Preceding the Model 299/B-17 on the design board but not to the air, Boeing’s Model 294/XB-15 was built in response to the USAAC’s early 1930s Project A requirement for a heavy bomber able to fly 5,000 miles while carrying a one-ton bomb load. This was a very ambitious set of specifications, given that the Air Corps was only then just phasing in its first monoplane bomber, the Martin B-10. Indeed, it was not until the advent of the B-36 that the USAAF could field a bomber capable of such combat radius unrefueled.
First flown in October 1937, the XB-15 had been designed with Allison V-3420 liquid cooled engines, but delays with that powerplant forced the use of R-1830s; as the -1830 was little more than third as powerful as the Allison engine, the XB-15′s designed top speed of 200mph was never to be achieved.