Staff Writer (Updated: 2/27/2016):
The B-10 originated as a private venture initiative by the Glenn Martin Company through the "Model 123" design. This work then begat the XB-907 evaluation models for the U.S. Army and featured Wright SR-120-E radials as well as open-air crew positions. These were seen in April of 1932.
The XB-907A was a revised form incorporating several requested changes. Modifications were made to the wing mainplanes to widen their reach. Engines consisted of 2 x Wright R-1820-19 series radials.
The XB-10 marked official prototype forms for the U.S. Army and brought about the use of enclosed crew positions and revised undercarriage legs. Evaluation models then became YB-10 and YB-10A, the latter shifting from R-1820-25 radials to R-1820-31 turbo-supercharged radials. Fourteen YB-10s were produced to the single YB-10A that was completed.
Once in production (as the "B-10") and reaching operational service status in June of 1934, the B-10 became the first quantitative American bomber design of all-metal construction and the first anywhere in the world to outpace pursuit fighters of the period. It was also the first local design to feature turreted armament for defense against intercepting enemy fighters. For the United States, the B-10 became a breakthrough design that helped to lay down doctrine, practices, and lessons for future bomber needs still to come.