It began initially as a private venture with the Martin Type 123, by Glenn L. Martin Company in Baltimore, Maryland. It was manned by a crew of four, pilot, co-pilot, nose gunner and fuselage gunner. At first, all four crew compartments were open, but it had a number of design innovations as well.
Innovations included a deep belly for an internal bomb bay and a retractable landing gear. The Type 123 was powered by two 600 hp Wright SR-1820-E Cyclone radial engines and flew for the first time on 16th February 1932. It was delivered to the US Army on 20th March as the XB-907 for testing. After evaluation by the US Army, it was sent back to Martin for redesign and improvement and it was rebuilt as the XB-10.
The XB-10 was later delivered to the US Army and it differed in many ways from the original aircraft. It had full NACA cowlings to decrease drag, while the XB-10 had Townend rings. It also was powered by two 675 hp Wright R-1820-19 engines and had a larger wingspan, together with an enclosed nose turret. When further trials were performed on the XB-10 in June 1932, it achieved a speed of 197 mph (317 km/h) at 6.000 ft (1.830 m) high, quite an impressive performance for the time.
After the success of the XB-10, some changes were made to the design. The number of crew members was reduced to three and canopies for all of them were added. The US Army ordered an initial batch of 48 of these on 17th January 1933. The first 14 of them were labelled as YB-10 and were delivered to Wright Field, Ohio, from November 1933 onwards and used in the Army Air Corps Mail Operation.
Later, in 1935, the Army ordered an additional 103 aircraft, named as the Martin B-10B, which differed only in minor details from the YB-10. Deliveries began in July 1935 and they served with the 2nd Bomb Group at Langley Field, Virginia, the 9th Bomb Group at Mitchel Field, New York, the 19th Bomb Group at March Field, California, the 6th Composite Group in the Panama Canal Zone and the 4th Composite Group in the Philippines. Some modified YB-10s and B-12 (the remaining aircraft of the original order, which were powered by two Pratt & Whitney R-1690-11 Hornet radial engines) were operated with twin floats for coastal patrol duties.
In February 1936 thirteen B-10Bs belonging to the 49th Bomb Squadron dropped supplies to the residents of Tangier Island, in Virginia and Smith Island, in Maryland as ships couldn't reach due to heavy ice in the Chesapeake Bay and the inhabitants faced starvation after a sever winter storm.
Although at the time of its creation it was a very advanced design (General Henry H. Arnold referred the bomber as the airpower wonder of its day), rapid advances in bomber design during later 1930s meant that the bomber was completely obsolete by the time the World War 2 began. A modernisation effort was made with the Martin 146, but it was superseeded by the Boeing B-17 and the Douglas B-18.
Ironically, although by the time the USA entered the war the B-10 was retired from active service long ago, an export model used by the Dutch East Indies escaped to Australia on 7th March 1942 and was taken on strength by the USAAF to serve as an utility aircraft. That was the only Type 139WH-3/Type 166 to serve with the United States Army Air Forces.
Sources:
1. https://en.wikipedia.org/wiki/Martin_B-10
2. http://www.joebaugher.com/usaf_bombers/b10.html
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