In response for the Army's request for a large, multi-engine bomber, the Model 299 (B-17) prototype, financed entirely by Boeing, went from design board to flight test in less than 12 months. The B-17 was a low-wing monoplane that combined aerodynamic features of the XB-15 giant bomber and the Model 247 transport.

The B-17 was the first Boeing military aircraft with a flight deck instead of an open cockpit and was initially armed with bombs and five .30-caliber machine guns mounted in clear "blisters."

It took to the skies for the first time on July 28, 1935.