Every now and then an aircraft comes along that redefines the standard from which all
future types would be built, in the case of American bomber design in the 1930s this would
come in the form of the Martin B10.
By the end of the previous decade, America’s bomber force was starting to look not only
outdated, but unable fulfil the requirements of a rapidly evolving air force.
All operational bombers were the tried and tested, but slow, biplane designs – most
of which were members of the Keystone series – and these were quickly showing their obsolescence.
Sleek monoplane designs were already starting to appear in the commercial aviation sector,
particularly in Europe, but these had thus far struggled to gain favour in many military
circles.
Though the benefits of the monoplane had been demonstrated, the US Amy and Navy still required
all military planes to be built as biplanes.
This was partly due to conservatism, and partly because it was thought that their reliability
made them cheaper – in actuality, the monoplane had become cheaper to both fly and maintain
by 1929, but senior officers at the time didn’t like being told that they were wrong.
Eventually, and sometimes with great reluctance, it was accepted that the monoplane was the
future, and the Army Air corps began seeking designs for a new monoplane bomber.
This quickly resulted in the development of several experimental aircraft.
Douglas came up with the gull-winged XB7, Fokker developed the short lived XB-8, and
Boeing seemed to be the strongest contender when they produced the YB9.
All of these were an improvement on previous biplane models, but each design was a compromise
of old thinking with new technology.
Watching this from afar, Martin began work on a private venture of their own.
They had previously built bombers for the Amy Air Corps, notably the successful MB-2
of the mid-1920s, and they now worked on a design to not only beat the competition but
render them totally obsolete.
The prototype, designated as the Model 123, was a rapid departure from anything Martin
had developed before.
It was a sleek, all-metal, cantilever mid-wing monoplane of advanced construction.
The wing was built in three sections, a centre section built into the fuselage which carried
the two engine nacelles in the leading edge, and two detachable outer sections.
All of these had a structure of riveted aluminium with stressed-steel fittings, and the whole
lot was covered in a sleek stressed aluminium skin.
Like the wing, the all-metal fuselage was also built in three sections, however its
metal skin had a different arrangement.
The sides of the fuselage were covered in a smooth skin of stressed aluminium to deal
with the sheer loads during flight, whereas the top and bottom of the fuselage was clad
in corrugated skin that could better handle compression loads.
Unlike previous bomber designs, which usually carried their drag-inducing bombs under the
wing and fuselage, the model 123 was the first to contain the entirety of its bomb load in
an internal bomb bay.
This was not only a practical innovation, but one for safety as well, as carrying fused
bombs within the plane usually had better safety margins than those stored externally.
It also allowed the bombardier control of when the bombs would be armed, depending on
the bomb and fuse type.
Another featured innovation was the provision for a mechanically retractable landing gear,
which again was done for the sake of streamlining.
However, like the internal bomb bay, this would come at a cost of added weight, and
as such the bomber would be equipped with a pair of 600hp Wright SR-1820 Cyclone engines.
Keeping up with the trend, this too was an innovation as the Cyclone was still in its
earliest stages of development, and its successful use in the Martin prototype was a contributing
factor in it being use in other aircraft of fame later on in its life.
The prototype was completed and delivered to the Army on March the 20th 1932, who upon
receiving it sent the plane to Wright Field for evaluation.
At this point the aircraft was given the new designation of XB-907.
In its completed form, it had a wingspan of just over 62 feet, and with a new pair of
streamlined engine cowlings it was expected to achieve a top speed of at least 190 mph.
Depending on what source is cited, it either had a crew of three or four, the point of
debate is whether it initially had provision for a co-pilot or not as it was later removed
for the production model.
Regardless of number, all of the crew positions were open to the elements in this first design,
which considering the hoped-for top speeds would have made for a frigid experience on
that crisp March morning in central Ohio.
Frozen pilot aside, the prototype performed better than expected during its first flights.
Not only did it take off and land in a shorter distance than expected, courtesy of modern
split-flaps, but it also reached a top speed of 197 mph at 6,000 feet.
In light of these promising figures, the prototype was sent back for modifications in the Autumn
to further improve its capabilities.
The nose went through an extensive re-design, which included the fitting of the first enclosed
gun turret to ever be found on a US military aircraft.
This turret was manually rotated by the gunner and featured a single 30 calibre browning
machine gun.
Though it did somewhat reduce pilot visibility for take-off and landing, it was believed
that this was offset by the nose gunner themself, who in a pinch could provide visual guidance.
Though I can’t imagine that would have been a particularly fun affair for anyone involved,
especially in poor weather.
Other modifications included the change from the shorter Townend Ring cowlings to the more
encompassing NACA units, an upgrade in engine to the newer 675hp version of the Cyclone,
and the widening of the wingspan by a further 8 feet.
Though all of this added another 2,000 lbs to the total weight of the aircraft, it performed
even better when it returned to Wright Field in October 1932, reaching a new top speed
of 207 mph.
As a result of its outstanding performance, the Army Air Corps placed a production order
for 48 bombers and purchased the original prototype, redesignating it as the XB-10.
Between that moment and the first deliveries of the production B-10s some additional changes
were made to the overall design.
It was accepted, and in no small part thanks to some very vocal complaints, that a plane
flying over 200mph should not have open crew stations, and so the pilot’s cockpit and
rear gunner’s station were finally enclosed.
The number of crew was also reduced to three, and the weight saved by ditching the co-pilot
was used to allow the installation of radio equipment that would be used by the rear gunner.
Various engine installations were made to the 48 units delivered for the initial production
order.
The first 14 aircraft, designated as the YB-10 had a similar version of the Cyclone engine
that powered the first prototype, and thus their overall performance was very similar.
Most of the production order would be powered by a more powerful version, giving out 775hp,
and these units would be delivered as the B-10b.
There was also an experimental model, the YB-10A, which featured turbo-supercharged
versions of the engine that gave it a top speed of 236 mph at 25,000 feet, however for
various reasons – but mostly due to cost- only one of these were built.
When it entered service, the B-10 was faster than any fighter currently operated by the
US military, and its advanced performance rendered all other bomber designs obsolete
overnight.
Domestically they would serve with numerous air groups, both as a conventional bomber
but also in some auxiliary roles.
In January of 1931 the Army Air Corps assumed responsibility for Coastal defence around
the U.S mainland.
This was the source of considerably grumbling, as this task had previously been the cherished
prerogative of the US Navy.
As such, the Army found several Navy-friendly aircraft manufacturers a bit reluctant to
help them out, and the decision was made to adapt several models of the YB and B10 into
the YB12.
These aquatic conversions usually entailed the fitting of large floats for water use,
but for the northern regions there was also the use of a ski-and-wheel landing gear.
Aside from this particular task, the B10 sometimes found itself involved in events that caught
the general eye of the public as well.
A notable example of this occurred in 1936, when a group of B-10s were used to drop much
needed supplies to island communities in the Chesapeake Bay.
These islands had been cut off after a particularly nasty winter storm, which had iced up the
bay to such a degree that sending in supplies by ship was impossible.
In response to a request by the Red Cross, the 49th bomb group sent in several B6 and
B10 bombers for a re-supply mission.
Landing directly on the islands was deemed too dangerous, and so the planes conducted
the supply equivalent of a bombing run, sometimes dropping their supplies from a height of just
15 feet above ground.
The B10s made headlines again later that spring, when a flight of 9 aircraft made several long-distance
flights from Virginia to the Panama Canal.
Though this was not done in one single leg, with stops in Miami and Costa Rica, it did
much to display the projection of US airpower.
As a result of its success, the B10 was approved to be sold for export once Martin’s delivery
contracts for the Army had been completed, and in August of 1936, an example of the B-10
was demonstrated to foreign buyers as the model 139W.
Unsurprisingly, Martin found several customers who were very keen to purchase.
Eventually, models would be sold to Argentina, China, the Netherlands, the Soviet Union,
Thailand and Turkey.
Most of these units were exported as various versions of the model 139.
In general, they were very similar, with the main difference usually being a change in
powerplant.
The two biggest export customers for the B10 were Argentina and the Netherlands.
Argentina would eventually operate 39 aircraft across its army and navy, and the Netherlands
would operate 121.
They would also be the only customer for the updated version of the export model, dubbed
the 166.
This model was of the same basic design as the 139, but it featured a notable change
to the cockpit layout.
Instead of a separate pilot’s cockpit and rear gunner station, it now featured a long
glasshouse-style canopy.
Along with this, the nose and front turret fairing were modified to improve streamlining,
and the provision was made for additional bombs to be carried under the wing between
the engines and the fuselage.
By the time the B10s actually saw combat, there were already aircraft in production
that surpassed them.
However, because the B10 was so much of a leap forward compared to previous bomber designs,
the export models saw extensive use as they often still outclassed the aircraft operated
against them.
They got their trial by fire in 1937 , when five Chinese-operated B10s were flown on bombing
raids against Japanese ground forces around Shanghai and Nanking.
Though their speed made them effective, they were fielded in too few numbers to make a
large impact, and by the end of 1938 all of them had either been destroyed or damaged
beyond repair.
The remaining combat experience of the B10 was mostly found in the service of Thailand
and the Dutch East Indies, the latter forces seeing extensive operations against the invading
forces of Imperial Japan.
Operations varied between coastal patrol, anti-submarine efforts, and high-altitude
bombing.
Dutch operated B10s were sometimes used for low altitude missions, but a mix of strong
Japanese anti-aircraft fire, and some unfortunate friendly fire incidents, soon put an end to
that particular tactic.
Though it was a losing battle, the B10s played their part in holding up the Japanese invasion
as best they could, especially when it came to the defence of Sumatra.
As the end drew near, many of them would be used for reconnaissance duties to spot incoming
Japanese attacks thanks to their high speed, but by the end of 1942 Java was almost completely
surrounded.
Several B10s were shot down by Japanese fighters, many more were destroyed on the ground, and
the few remaining models were either captured or scrapped after the allied forces had surrendered.
Back home, the B10 would remain in service with Army bombardment squadrons until the
arrival of the B-17 and B-18 in the late 1930s.
During this time an attempt was made to build on the success of the B10 and design a competitor
against the new bombers.
The model 146 could easily be considered a fatter, albeit more powerful, version of the
B10, but in reality, it also featured a lot of brand-new components.
Besides being designed with a much longer range than the B10, it also featured improved
engines, fowler flaps, and a new cockpit that once again allowed for a co-pilot.
Though it was ultimately a failure, the model 146, along with its more successful forbear,
taught the designers at Martin many valuable lessons.
But surprisingly, despite over 340 being built, only a single example of the B10 survives
today.
An export model, originally operated by Argentina, and it can now be found at the National Museum
of the US Air Force, in Ohio.
By the time the last B10s were retired, having served the Royal Thai Air Force until 1949,
they had left the lasting legacy of the plane that reinvented American bomber design.
Traces of the B10s design can be seen in many famous US aircraft that were operated during
the second world war, and it directly influenced the design of Martins next, and perhaps most
famous fast bomber, the B-26 Marauder.
But that’s a story for another day, as always thank you all very much watching, and I’ll
catch you all next time.
Goodbye.