The Martin B-10 was the first all-metal monoplane bomber to be regularly used by the United States Army Air Corps, having entered service in June 1934.

Its all-metal monoplane airframe, along with its features of closed cockpits, rotating gun turrets (almost simultaneously with the 1933 British Boulton & Paul Overstrand biplane bomber's own enclosed nose-turret), retractable landing gear, internal bomb bay, and full engine cowlings, became the standard for bomber designs worldwide for decades.

[6] It also sported a pair of 675 hp (503 kW) Wright R-1820-19 engines, and an 8 feet (2.4 m) increase in the wingspan, along with an enclosed nose turret.

The first 14 aircraft were designated YB-10 and delivered to Wright Field, starting in November 1933, and used in the Army Air Corps Mail Operation.

In addition to conventional duties in the bomber role, some modified YB-10s and B-12As were operated for a time on large twin floats for coastal patrol.

Once the Army's orders had been filled in 1936, Martin received permission to export Model 139s, and delivered versions to several air forces.

[11] On 25 August 1937, as the air battles intensified in the early part of the Second Sino-Japanese War,[12] five Chinese Nationalist Air Force bombers of the 8th BG, 19th and 30th Squadrons consisting of three Heinkel He 111As and two Martin B-10s, flying from their base in Nanjing to Shanghai, successfully dropped their bombs on Japanese landing forces at Liuhe, Taicang, northwest of Shanghai.

[13][14] As the National Revolutionary Army of China fought desperately to hold onto their remaining positions in the Battle of Shanghai, the Chinese Air Force launched a major strike with a motley mix of aircraft against Japanese positions in Shanghai on 14 October 1937, consisting of three B-10s, two Heinkel He 111As, five Douglas O-2MCs, five Northrop Gammas, and three Curtiss Hawk IIIs from Nanjing in the late afternoon; in the evening, one bomber was launched every hour from Nanjing to attack Japanese positions in Shanghai until 03:00 on 15 October.

Hsu Huan-sheng and Lt. Teng Yen-bo, successfully flew the first air raid on mainland Japan; the unescorted nighttime raid over Japan saw the B-10s dropped 2 million leaflets in "alerting the conscience of the Japanese people against atrocities committed by the Japanese invasion and occupation of China", over the cities of Nagasaki, Fukuoka, Kurume, Saga, and others, while reconnoitering airbases, ports, warships and factories.

[20] Twelve Martin 139 WH-1s were ordered for the Royal Netherlands East Indies Army Air Force (ML-KNIL) in 1936, followed by 26 improved WH-2s in March 1937, sufficient bombers to equip a Group of three squadrons.

Due to a lack of coordination, British AA mistook the Dutch Martins for enemy aircraft and engaged them as they neared Singapore.

[25] In early January 1942, Dutch Martins along with British Blenheim bombers sortied over the west coast of Malaya to halt the Japanese advance.

[26] On 8 January, nine Martin and four Australian Hudson attacked a suspected Japanese seaplane tender anchored offshore in South China Sea, but the results were inconclusive.

[6] Rapid advances in bomber design in the late 1930s meant that the B-10 was eclipsed by the time the United States entered World War II.

The Model 139s in combat in China and South East Asia suffered the same disadvantages as the other early war medium bombers, i.e. not enough armor and guns, while it could not outrun the latest fighters.

However, it had a higher ceiling of 28,500 ft (8,700 m), was only 2 mph (3 km/h) slower, and carried 313 lb (142 kg) less in bombs than the Boeing, at over half the cost.