The Martin Model 139 was a twin-engined mid-wing monoplane bomber developed from the experimental Model 123 of 1932. During 1934 48 Model 139 were delivered to the USAAC. Fifteen were fitted with 503kW Wright R-1820 Cyclone engines as YB-10 and 33 with 577.5kW Pratt & Whitney R-1690 Hornet engines as YB-12 and B-12A, a number of which were later converted into twin-float seaplanes for coastal patrol duties.

Development of the Model 139 led to numerous improvements being made, including the installation of 551.4kW SGR- 1820-G3 Cyclone engines, a Sperry automatic pilot and the addition of wing flaps, constant-speed propellers, de-icers and numerous structural and maintenance refinements. During 1935 and 1936 103 of the improved aircraft (B-10B) were delivered to the USAAC.

On 1 July 1936 the bomber was released for export and was subsequently ordered by six foreign governments, the Netherlands East Indies alone receiving 120 Model 139W and Model 166 with 670.7kW Cyclone engines. The latter aircraft introduced improved aerodynamics and performance characteristics and featured a continuous cockpit enclosure instead of the previous separate enclosures.

3-View

A three-view drawing (752 x 901)