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Bristol Blenheim: The British Bomber

During the early years of World War II, twin-engine Bristol Blenheims filled a number of vital combat roles in Europe and various theaters around the globe. 

In the late ‘30s when they first flew, their performance was superior to most fighters then in production, though this advantage wouldn’t last long. 

Manufactured by Britain’s Bristol Aeroplane Company, Blenheim’s were built in large numbers, but due to inherent design flaws, relatively anemic engines and big advancements in aviation technology, their service lives were cut short in favor of more capable aircraft like Bristol’s speedier, more adaptable, and better armed Beaufighter. 

But though Blenheims are most well-known as warbirds, they were derived from civil airliners that were among the fastest and most advanced of their day. 

The concept was largely the brainchild of media magnate Lord Rothermere, but when the first civilian prototype took to the skies in April of 1935, the RAF took notice. 

For England the timing couldn’t have been better, because tensions were heating up across Europe and many of the planes in the RAF’s inventory were well on their way to becoming obsolete. 

Concept

Photo credit: RAF Museum. Young Frank Barnwell, source: wingstowar.info

In the early ‘30s Bristol’s head designer Frank Barnwell traveled to the United States to study the revolutionary twin-engine, low-wing monoplanes like Lockheed’s Electra 12A.

After returning to England he produced a design that was in many respects nearly identical to the American airliner. 

With similar fuselage, wings and powerplants, Barnwell was convinced that his new plane’s performance would be phenomenal.

The following year Lord Rothermere, owner of the Daily Mail, challenged Barnwell to up the ante by producing the world’s fastest civil airliner, one capable of carrying six passengers and two crewmen at speeds approaching 300 miles per hour (482 km/h). 

At the time German aircraft manufacturers were already producing record-breaking machines like Heinkel’s single-engine He 70, but though Rothermre fancied a private executive aircraft, he wasn’t keen on buying a German plane. 

In addition, a domestically produced state-of-the-art aircraft would reestablish Britain’s technological superiority and help grow the country’s fledgling civil aviation industry, which everyone agreed would spur economic growth. 

After consulting with Bristol’s design team Rothermere wasn’t satisfied with initial performance estimates, but with the addition of more powerful supercharged Bristol Mercury engines, the Blenheim could conceivably be a truly one-of-a-kind machine.  

In the spring of 1934 Rothermere ordered a single aircraft designated Type 142, and a year later the new plane barrelled down the runway and climbed into the sky over Filton Aerodrome in South Gloucestershire.

To Rothermere’s delight, his 142 proved to be even faster than expected, hitting nearly 310 miles per hour (498 km/h) in level flight.  

A few months later the Air Ministry officially expressed interest in the aircraft and began drawing up specs for a military variant that would primarily be used as a medium bomber, though it could potentially serve in various other roles as well. 

Design

Bristol 142M Blenheim cutaway by kitchener.lord is licensed under CC-BY-NC-ND

Blenheims were among the first British aircraft to incorporate stressed-skin construction, retractable landing gears, and powered gun turrets. 

Originally powered by Bristol Mercury VIII air-cooled radial engines producing about 850 horsepower each, power was sent to another relatively new innovation – three-blade variable pitch propellers – which optimized performance at various altitudes. 

To achieve their high speeds, Blenheims also featured narrow fuselages and “stepless cockpits” that helped them slip through the air more efficiently, but on the downside the crew compartment was hopelessly cramped and visibility was poor.  

Blenheims were 42 feet 7 inches (12.98 m) long, 56 feet 4 inches (17.17 m) from wingtip to wingtip, and capable of taking off at weights exceeding 14,000 pounds (6,350 kg). 

Fuel was supplied by two 140-gallon internal tanks located along the fuselage’s centerline, and to speed construction and reduce maintenance times, engines were housed in split nacelles protruding from the leading edge of each wing. 

In addition, much of the airframe and wing components were prefabricated on the factory floor, after which they were bolted and riveted together. 

For additional weight savings, elevators in the tail consisted of metal frames covered with fabric. 

Though the undercarriage was lowered and retracted hydraulically, the system could be operated manually in case of battle damage or mechanical issues. 

Blenheim crews typically consisted of a pilot, bombardier/navigator and a radio operator/ gunner, with the pilot’s station located on the left side of the nose. 

However conditions were so cramped that many vital gauges were hidden behind the control yoke, and even worse, the propeller pitch controls were behind the pilot, which meant that they had to be manipulated by feel alone. 

Armament initially consisted of a single forward-firing .303 in (7.7 mm) Browning machine gun housed in the wing just outside the port engine, as well as .303 Lewis Gun in a semi-retracting dorsal turret pointing rearward.  

Blenheims could carry about 1,000 pounds (450 kg) of bombs in their internal bays, but like many British aircraft of the era the doors weren’t operated electrically or hydraulically. 

Instead, before bombs were dropped the doors had to be unlatched manually, after which the weight of the bomb would open them. 

This gravity-based system saved weight but was cumbersome and inefficient, and when the bomb bay doors didn’t open instantly, even short delays could result in the ordinance missing its target, sometimes by hundreds of yards. 

Production

In September of 1935 Bristol received an order for 150 aircraft, despite the fact that the only prototype that had been built was an airliner. 

Nonetheless, modernizing the RAF was a top priority, and production proceeded so rapidly that the new bomber’s maiden flight took place less than a year later in June of 1936.  

By the end of 1936 more than 1,500 Blenheims had been built, many of which had already been distributed to units in England and abroad. 

As domestic orders poured in, additional assembly lines were set up to meet the demand, and many aircraft were built under contract by Avro and Rootes Securities. 

At the same time international orders were increasing as well, and Bristol licensed manufacturers in Canada, Finland and Yugoslavia to build Blenheims on their own.

Production of the first Mark I variant ended in 1939 in favor of updated versions that would go on to serve in various roles like anti-shipping and maritime reconnaissance. 

To accommodate additional equipment like radar, later variants did away with the stepless cockpit, which increased cabin spaces, improved visibility, and gave the new airplanes a more aggressive look. 

To extend range, reconnaissance variants held nearly 60% more fuel, but though Mark IIs had more powerful engines, their added weight negated expected gains in performance.  

Other variants included interceptors and night fighters, each of which was fitted with a gun pack that included four .303 caliber Browning machine guns housed in a pod protruding from the bottom of the fuselage slightly behind the cockpit. 

The last ground attack variants included strengthened airframes, armored cockpits, four more Brownings in the nose, and even more powerful engines producing about 950 horsepower each. 

Later variants were superior to early versions in many respects, but their low power-to-weight ratios meant that they were particularly vulnerable to ground fire and enemy fighters. 

Mark IV production continued until mid-1943, after which Blenheims were steadily phased out as more Beaufighters became available. 

Blenheims in Action

Blenheim IV by Geoff Collins is licensed under CC-BY-NC-ND

Honoring its commitment to Poland’s sovereignty, Great Britain declared war on Nazi Germany in early September of 1939. 

Already in service in far away locales like Egypt, India and Singapore, the very day war was declared a Blenheim Mark IV piloted by Andrew McPherson became the first British aircraft to cross into German air space on a high-altitude reconnaissance mission over Wilhelmshaven

The following morning more than a dozen Blenheims attacked warships that’d been spotted the previous day. 

The RAF also positioned a number of Blenheims at French airfields, from which they carried out shorter range missions against various German targets, but though they enjoyed some success, losses to enemy ground fire and fighters were staggeringly high, and many more were destroyed on the ground.  

In one instance in the middle of May 1940, of 71 aircraft that set out to attack a large German force, 40 failed to return home, marking one of the highest single-day debacles in RAF history. 

Losses never again reached such epic proportions, but even when escorted by capable British fighters, the rate at which Blenheims were being destroyed was excessively high. 

Unfortunately for Blenheim crews there were never enough escort fighters, and their aircraft’s lack of maneuverability and relatively poor performance made them easy pickings, despite the fact that they were defended by both tail and top-mounted turret guns. 

Between the end of May and early June of 1940, more than four dozen Blenheims supported the massive evacuation at Dunkirk, though by then each of the remaining squadrons was operating with far fewer aircraft than was standard.  

By the following year many Blenheims had been relegated to night fighter duties where speed and maneuverability weren’t so crucial, and where encountering enemy fighters wasn’t as likely. 

In the German night-bombing raids on London in mid-June of 1940, Blenheims were credited with shooting down nearly a dozen bombers, and they were the first aircraft to attack Italian positions after the country entered the war on the Axis side. 

As the war progressed more and more Blenheims were repositioned to North Africa and the Mediterannean where they primarily carried out reconnaissance, bombing, and anti-shipping missions in support of ground and naval forces. 

Later in December 1941 many Blenheims and their crews were again repositioned to the Pacific theater to counter new threats from Japanese forces, where they were particularly effective at destroying ground targets and aircraft sitting idly on tarmacs. 

End of the Blenheim

Even by military aircraft standards Blenheims had surprisingly short service lives. 

Huge advances in aviation technology were made during the late ‘30s and early ‘40s, and by the time they began rolling off the assembly lines they were largely outclassed by both Allied and Axis aircraft.   

But despite their inherent drawbacks, Blenheims continued to operate until about 1943, mostly in India, Singapore, British Malaya, and the Dutch East Indies. 

By the end of the war most were strictly used as trainers, having been replaced by Bristol Beaufighers and legendary de Havilland Mosquitos. 

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