What military type aircraft, known as "Model 299" was the prototype for one of World War II's most successful bombers?

The Boeing Model 299, which was introduced in 1935, was a one-of-a-kind prototype bomber, which so favorably impressed officers of the U.S. Army Air Corps, that a modified version was built and delivered as the B-17 "Flying Fortress." The Model 299 is often referred to as the XB-17, but never officially was designated as such. The Model 299 crashed shortly after takeoff for a test flight, due to a crew error. All of the production B-17s, of which 12,731 were built, also carried the Boeing Model 299 designation as well as the B-17 type designation. The B-17 was one of the most successful U.S. bombers of World War II and served in both the European and Pacific combat areas. The name "Flying Fortress" is a reference to the proposed use of the bomber in coastal defense rather than as an offensive weapon.