The Martin B-10 was the first all-metal monoplane bomber to enter full
production for the US Army. It was also the first bomber to have a
performance that exceeded that of contemporary pursuit aircraft.
The immediate ancestor of the B-10 was the Martin Model 123, which was designed and built as a private venture by the Glenn L. Martin Company of Baltimore, Maryland. It was a midwing all-metal monoplane. The monocoque fuselage had corrugated top and bottom surfaces with a deep belly. The deep belly carried doors for an internal bomb bay, so the bombs could be carried internally rather than on external racks as in the Boeing YB-9. The main landing gear retracted backwards into the rear of the engine nacelles, but the lower half of the wheels remained exposed. Four crew members were to be carried. Three of the crew members were seated in separate open cockpits on the top of the fuselage. The nose gunner/bombardier had a transparent aiming position in the lower nose, the pilot sat in an open cockpit abreast of the forward wing, and the rear gunner sat in an open position in the rear dorsal fuselage. The fourth crew member occupied a position inside the fuselage.
The Model 123 flew for the first time at Baltimore on February 16, 1932. It was powered by a pair of 600 hp Wright SR-1820-E Cyclone engines that were enclosed by NACA low-drag cowling rings. The wingspan was 62 feet 2 inches.
The Model 123 was delivered to the Army on March 20, 1932 under a bailment contract. Although still Martin property,
the aircraft was assigned the designation XB-907 for its trials at Wright Field. Trials began in July of 1932.
During the trials at Wright Field, a maximum speed of 197 mph was recorded at an altitude of 6000 feet. This was a truly
spectacular performance for 1932, and faster than pursuit aircraft in service at the time.
In mid-1932, the Model 123 was returned to the factory in Baltimore for some suggested modifications. During the early autumn of 1932, the open-cockpit gun position in the nose of the Martin 123 was replaced by a gun turret. This was a transparent, manually-rotated facility, equipped with a single 0.30-inch machine gun. The pilot's cockpit and the dorsal gunner position remained open. At the same time, more powerful 675 hp R-1820-19 Cyclone engines were installed. These engines were also fitted with full cowlings that extended forward of the wings. A new longer-span wing was fitted, increasing the wingspan to 70 feet 7 inches. The designation was changed to XB-907A when it was returned to Wright Field for more tests.
Trials of the XB-907A took place at Wright field in October of 1932. Despite an increase of nearly 2000 pounds in the gross weight to 12,230 pounds, the XB-907A had a maximum speed of 207 mph at 6000 feet. The Model 123 was a truly revolutionary design, and every other bomber in the world (and just about every pursuit plane as well) was instantaneously made obsolete. In 1932, the Glenn L Martin company was awarded the Collier Trophy for its work.
On January 17, 1933, the Army purchased the XB-907A under the designation XB-10. The serial assigned was 33-139. At the
same time, the Army ordered 48 production examples of the Martin design. These were designated Model 139 by the factory.
The first 14 aircraft were designated YB-10 (33-140/153). They were
powered by 675 hp Wright R-1820-25 engines. They differed from the
prototype primarily in having transparent sliding canopies fitted over
both the pilot's cockpit and the rear gunner's position, a concession
to the 200 mph-plus speeds that could be attained. The rear cockpit
was modified to accommodate a radio operator in addition to the
gunner. Armament consisted of a 0.30-inch Browning machine gun in the
nose turret, a 0.30-inch gun in a flexible position in the dorsal
position, plus a 0.30-inch machine gun in a tunnel position in the
fuselage floor behind the bomb bay to guard against attacks from
below. The internal bomb bay could carry two 1130-pound bombs or five
300-pound bombs. There were provisions for an external shackle under
the right wing for a single 2000 pound bomb. The YB-10 could be
distinguished from its successors by the presence of an oil cooler
scoop on top of the engine cowling.
The first YB-10 was delivered to Wright Field in November of 1933. Most of the YB-10s were based at March Field in California with the 7th Bomb Group until December of 1934, when it re-equipped with B-12s. The YB-10s then remained at March Field with the 19th Bomb Group. In a demonstration of their reliability and efficiency, ten YB-10s undertook a survey flight to Alaska in July of 1934.
A single YB-10A (33-154) was included in the 48 aircraft of the
original order. It was delivered in June of 1934 with a pair of
experimental turbosupercharged R-1820-21 Cyclones. It achieved a
maximum speed of 236 mph at 25,000 feet. Despite the high performance
that was achieved, the turbosuperchargers were not sufficiently
reliable to be introduced into production models. Consequently, there
was no B-10A production model, the first production model of the B-10
series being the B-10B.
Another experimental version included in the original order was the
XB-14 (33-162), which was similar to the YB-10 but was powered by a
pair of 950hp Pratt & Whitney YR-1830-9 Twin Wasps. It was common in
those days to assign a separate model number to aircraft which
differed from each other only in the type of engine which powered
The remaining 32 aircraft on the original order were delivered as
B-12As. They differed from the YB-10 primarily in being powered by
Pratt and Whitney R-1690-11 Hornet radials in place of the Wright
Cyclones. Despite their new model number, they were otherwise quite
similar to the YB-10. They will be described more fully under the B-12 entry.
Production of the Martin bomber was continued by FY 1934 and 1935 Army procurements for 103 examples of the B-10B, the primary service version. This was the largest procurement of bomber aircraft since World War I. The B-10B could be distinguished from the YB-10 by the presence of air intakes on top the nacelle as well as by the relocation of the exhaust pipes from the lower nacelle to outlets at the nacelle top immediately behind the air intakes. It was otherwise quite similar to the service test YB-10.
The first B-10B arrived at Wright Field in July of 1935. Production deliveries to Langley Field began in December of 1935 and were completed by August of 1936. The B-10B served with the 2nd Bomb Group at Langley and the 9th at Mitchell Field. The B-10B served (along with YB-10s) with the 19th Bomb Group based at March Field in California. The B-10B also served with the 6th Bomb Group based in the Canal Zone, and was issued to the 28th Bomb Group based in the Philippines.
Army records do show that two aircraft designated simply B-10 were ordered as part of the the original B-10B contract, with serials 36-347/348 being assigned. They may have been replacements for the two experimental models (YB-10A, XB-14), but it is uncertain if these were actually delivered and, even if they were, in what particular configuration.
In January of 1931, the US Army was assigned the responsibility for coastal defense around the United States mainland. As part of this mission, several Army YB-10s were temporarily fitted with large floats for water-based operations.
The B-10s remained in service with Army bombardment squadrons until
the advent of the B-17 and B-18 in the late 1930s. The advances in
bomber technology suddenly became so rapid that the B-10,
revolutionary though it was, swiftly became obsolete as the 1930s
progressed. By 1940, the B-10B was thoroughly out of date and had
been largely relegated to secondary roles such as target towing. No
US Army B-10Bs participated in any combat during World War 2.
With such an advanced performance, the Martin company fully expected that export orders for the B-10 would come flooding in. However, since the Army owned the rights to the Model 139 design, it forbade any export overseas until its own orders had been filled. However, following the completion of the last example for the Army in 1936, clearance was finally given for the export of the Martin bomber.
The first export demonstrator, the Model 139W, was completed in August of 1936. It was powered by a pair of 750 hp Wright R-1820-F53 Cyclones. The civilian registration number NR-15563 was applied. It was sent to Argentina in September to compete against the German Junkers JU-86 and the Italian Savoia SM-79B for Argentine orders. The Martin plane won the contract, and Argentina ordered 13 examples for the Navy and 26 for the Army.
Other export orders soon followed. The Martin 139WC was a version intended for China. It was powered by a pair of 850 hp R-1820-G2 Cyclones. 6 examples went to China in February of 1937. They were used in combat when Japan invaded China in August of 1937. These Martin 139WCs were the first American-designed bombers to see combat. However, the results were not all that good, since most were destroyed on the ground during Japanese air attacks.
Six Martins were sold to Siam in April of 1937. They were powered by R-1820-G3 Cyclones. 20 Model 139Ws were sold to Turkey in September of 1937, powered by R-1820-G2 engines
A single Model 139WR (X16706) was sold to the Soviet Union for evaluation. Its fate is unknown.
Plans to sell Martin 139s to Republican Spain were blocked by the State Department. Reports that the Martin bomber was being used in Spain on the Republican side were in error, being misidentifications of the Soviet Tupolev SB-2, which was basically similar in overall configuration.
The largest customer for the export Martin bomber was the Dutch East Indies. The first Dutch order was for 12 Model 139WH-1 bombers powered by 750 hp R-1820-F53 Cyclones. They were delivered between September 1936 and February 1937. 26 Model 139WH-3s, powered by 840 hp R-1820-G3s were delivered from November 1937 to March 1938. The final export version was the Model 139WH-3 (or Model 166), powered by a pair of 900 hp Wright R-1820G-102 radials. It had a long unbroken transparent canopy "greenhouse" that extended from the pilot's cockpit all the way to the rear gunner's position. 78 of these new bombers were delivered by May 5, 1939, when the last export Martin bomber rolled off the Baltimore production line.
Between mid-1936 and 1939, a total of 189 export Model 139W and Model 166 bombers had been manufactured.
Six squadrons of Martin bombers were serving in the Dutch East Indies when the Japanese invaded. Dutch crews flew these Martin bombers in a futile attempt to stem the Japanese advance into the Dutch East Indies during early 1942. By this time, the Martin bomber was thoroughly obsolete, and its speed and armament were completely inadequate to protect against the fast and heavily-armed Japanese Zero fighters. Most were shot down in combat or were destroyed on the ground.
A surviving export Model 139 fled from the Dutch East Indies to Australia on March 7, 1942. It was taken on strength by the USAAF for use as an utility aircraft and assigned the serial number 42-68358. This was the only export Martin 139 to serve with the USAAF.
An Argentine Martin Model 139 was returned to the USA in 1976. It was refurbished as a standard USAAC B-10B and is now on display in the Air Force Museum at Wright-Patterson AFB in Ohio. So far as I am aware, it is the only surviving Model 139 bomber.
33-139 Martin XB-10 33-140/153 Martin YB-10 33-154 Martin YB-10A 33-155/161 Martin YB-12 33-162 Martin XB-14 33-163/177 Martin B-12A 33-258/267 Martin B-12A 34-028/115 Martin B-10B 35-232/246 Martin B-10B 36-347/348 Martin B-10 (not certain that these were delivered) 42-68358 Martin B-10 (ex-Dutch Model 139 impressed by USAAF)
Specification of Martin B-10B:
Two Wright R-1820-33 Cyclone air-cooled radial engines, rated at 775 hp for takeoff and 750 hp at 5400 feet. Maximum speed 213 mph at 10,000 feet, 196 mph at sea level. An altitude of 5000 feet could be attained in 3.4 minutes. Cruising speed 193 mph. Landing speed 65 mph. Service ceiling 14,200 feet. Normal range 590 miles, maximum range 1240 miles, ferry range 1830 miles. Weights: 9681 pounds empty, 14,600 pounds gross, 16,400 pounds maximum. Dimensions: wingspan 70 feet 6 inches, length 44 feet 9 inches, height 15 feet 5 inches, wing area 678 square feet. One 0.30-inch Browning machine gun in nose turret, one 0.30-inch Browning machine gun in flexible mount in dorsal gunner position, and one 0.30-inch Browning machine gun in a ventral tunnel position mounted in the floor of the fuselage behind the bomb bay. 2260 pounds of bombs could be carried.