The M.20 was a Cheap and Cheerful Spitfire Backup

Designed by Miles Aircraft Limited in 1940, the M.20 was conceived as an emergency fighter that could be quickly and cheaply produced in the event of significant losses of frontline Royal Air Force (RAF) fighter aircraft, such as the Supermarine Spitfire and the Hawker Hurricane.

Miles actually managed to produce a very capable machine in an extremely short amount of time. It had comparable speed and firepower to the Hurricane and Spitefire, but was much cheaper and simpler to make.

Despite its potential, the M.20 never saw combat. It briefly dabbled with the Fleet Air Arm as a disposable fighter, but it didn’t succeed there either.



In the summer of 1940, Britain found itself standing alone against Nazi Germany after the fall of France. The German Luftwaffe launched a concerted air campaign, aiming to gain air superiority over the British Isles as a precursor to a possible invasion, known as Operation Sea Lion. This would result in the famous Battle of Britain.

The Battle of Britain, fought between July and October of that year, saw the Royal Air Force (RAF) defending the United Kingdom against the German Luftwaffe’s sustained aerial bombardment, aimed at achieving air superiority as a precursor to a possible invasion.

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The battle was characterized by intense dogfights over the English Channel and British soil, involving iconic aircraft such as the Spitfire and Hurricane on the British side, and the Messerschmitt Bf 109 on the German side.

Spitfire trails smoke after being hit while attacking German Heinkel He-111 bombers, seen from the cockpit of an He 111, Battle of Britain, September 1940
Spitfire trails smoke after being hit while attacking German Heinkel He 111 bombers, seen from the cockpit of an He 111, Battle of Britain, September 1940

It was critical for the United Kingdom that they were able to prevail through the battle to victory, as a loss would mean the nation would lose air superiority and be left open to a large-scale invasion by Germany. Germany planned to mount an invasion once the RAF had been removed as a threat.

The Battle of Britain was characterized by intense dogfights over the English Channel and British soil, involving iconic aircraft such as the Spitfire and Hurricane on the British side, and the Messerschmitt Bf 109 on the German side. Both sides took great losses as they tried to eliminate each other.

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During this period, there was a genuine concern that Britain’s fighter production could not keep pace with the losses being incurred. The RAF’s frontline fighters were proving effective but were complex and time-consuming to manufacture. Additionally, there was the ever-present threat of German bombing campaigns targeting aircraft factories, which could further reduce Britain’s capacity to produce these vital aircraft.

An early Spitfire Mk IIA. Note the 3 blade prop.
British production of its fighters was of desperate importance. Spitfires and Hurricanes were needed to defend the Luftwaffe’s attacks.

The Air Ministry’s Request

Faced with these challenges, the Air Ministry issued Specification F.19/40 in July 1940, calling for the design of a simple, easy-to-build fighter aircraft that could be rapidly produced using non-strategic materials. It was required to reach 350 mph and an altitude of 32,000 ft.

This aircraft was essentially a back up in case the production of Spitfires and Hurricanes was affected or could not meet demand. The specification outlined the need for an aircraft that could perform comparatively with those aircraft, and powered by the Rolls-Royce Merlin engine, which was already in use in both the Spitfire and Hurricane, to simplify logistics and manufacturing.

Miles M.20/1 mock.
A mock-up M.20/1, an earlier plan for a cheap fighter created by Miles before a requirement had even been considered.

Interestingly, the US began a similar project around the same time to produce a fighter that was as simple and cheap to make as possible. This aircraft, the Bell XP-77, was incredibly small and made entirely out of wood (except its skin).

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Miles Aircraft’s Response

Miles Aircraft, led by Chief Designer Frederick George Miles, responded to this specification with remarkable speed and ingenuity. Founded by Frederick and his wife Blossom, the company initially operated under the name Phillips & Powis Aircraft Ltd. before becoming Miles Aircraft Ltd. in 1943.

The company was known for their innovative designs and the use of wood in aircraft construction, and already made light civilian and military trainer aircraft, so they embraced the challenge of creating a fighter that could meet the RAF’s needs under the constraints imposed by the war. In just 65 days the company designed and built their design, the M.20.


The design philosophy behind the M.20 was fundamentally different from that of the contemporary fighters it was meant to supplement. Instead of optimizing for maximum performance, the focus was on simplicity, ease of production, and the use of readily available materials.

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Miles proposed an aircraft that could be built by semi-skilled labor, using wood for the primary structure to avoid competing for strategic materials like aluminum, which were in short supply and high demand for other war efforts. The company had already drawn up a few designs for a similar aircraft, which enabled them to quickly respond to the requirements.

It was to utilise many components from the company’s Miles Master trainer, which was already being produced and serving in large numbers. The engine and its nacelle was identical to the those used on the Lancaster bomber, again, to simplify supply chains and make production easier.

The Miles M.20 on the ground.
The Miles M.20.

M.20 Design

Airframe and Structure

The M.20 featured a semi-monocoque construction, a technique where the aircraft’s skin supports most of the structural load. This method was well-suited to wooden construction, allowing for a relatively lightweight yet strong airframe.

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The choice of a fixed undercarriage further simplified the design and reduced the number of parts required, eliminating the need for complex retraction mechanisms and hydraulic systems. It also simplified the design of the wing itself, greatly reducing the overall complexity of the design.

Partially built M.20 fuselage.
The M.20’s wooden fuselage. Note the front windscreen, lacking the bubble canopy.

While the fixed undercarriage appeared obsolete at first, impacting the aircraft’s aerodynamic efficiency and top speed, it was actually a clever way to significantly speed up the production and maintenance processes.

On top of that, the fixed undercarriage was one-less system that had to be tested and refined before production, shortening the overall development time.

It was a rather small aircraft, measuring 30 ft (9.2 m) in length with a 34 ft 7 in (10.5 m) wingspan. Maximum take off weight was 7,800 lb (3,500 kg), slightly more than a Spitfire, but slightly less than a Hurricane. Overall it was physically similarly in size to those aircraft.

Miles prototype.
The aircraft was fitted with fixed landing gear for simplicity.


The M.20 was powered by a Rolls-Royce Merlin engine, the same type used in the RAF’s frontline fighters, the Spitfire and Hurricane, and the nation’s famous heavy bomber, the Lancaster. This choice was strategic, leveraging existing supply chains and maintenance expertise. The Merlin engine provided the M.20 with respectable performance, including a top speed that was competitive with more complex designs of the era, despite the aircraft’s simpler construction and fixed landing gear.

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The Merlin engine was a 27-litre, liquid-cooled V12 design. Early versions produced around 1,000 horsepower, but this number continued to increase throughout the Second World War as the engine was continuously developed.

M.20 in flight.
With a Merlin engine, the M.20’s top speed was around 330 mph.

One of the Merlin’s defining features was its supercharger, which was refined over several iterations to improve altitude performance. The two-speed, two-stage supercharger fitted to later models was a key factor in the engine’s exceptional high-altitude capabilities.

The M.20 used a Rolls-Royce Merlin XX that produced 1,260 hp. This engine gave the M.20 the power and performance it needed to hang with much more mature aircraft.


The proposed armament for the M.20 was eight .303 Browning machine guns, a standard configuration for British fighters at the time. This armament would have given the M.20 considerable firepower, allowing it to fulfill its role as a stop-gap fighter effectively. Impressively, its design allowed for up to 12 guns.

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This firepower put the M.20 on an equal footing to early Hurricanes and Spitfires. Interestingly, it could carry more ammunition than both of those aircraft (likely due to its thicker wings).

M.20 wing.
The M.20 carried eight machine guns, but there was space for 12.

Cockpit Canopy

Despite the simple, and sometimes archaic design elements of the M.20, its cockpit featured a very modern “bubble” canopy. At the time, contemporary fighters, including aircraft like the Bf 109 and P-40 Warhawk, featured heavily framed canopies that were blended into the fuselage.

This greatly limited visibility for the pilot. A bubble canopy sits above the fuselage, and gives excellent visibility all around. They wouldn’t become a standard until around half-way through the Second World War, but the M.20 featured one all the way back in 1940.

The M.20’s canopy slid back and forth for entry, and could be completely jettisoned to better facilitate and emergency exit.

M.20/ cockpit.
The cockpit was protected by 12 mm of bulletproof glass and 9 mm armor plates in front and behind.

Trials and Performance

After an extremely short development phase two M.20 prototypes were constructed, and Miles began testing in September 1940. Remember, Specification F.19/40 had only been issued in July – an incredible turn around!

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Test pilots found the aircraft to be pleasant to fly, with respectable performance characteristics. Its fixed undercarriage and thick wings increased drag, but it still could reach good speeds and was noted to have great acceleration.

Miles aircraft crash.
M.20 prototype after suffering a crash during trials.

It was able to reach speeds of around 330 mph, which is comparable to the versions of Spitfires and Hurricanes that were in service at the time. It was not as manoeuvrable as either of those aircraft, but considering its rapid development, it is an impressive feat.

However, with no pressing need for its mass production and the RAF’s focus on more advanced aircraft designs, the M.20 did not proceed beyond the prototype stage.


During testing one of the prototypes brakes’ locked up during landing, resulting in a non-fatal crash. Also around this time, the Battle of Britain was winding down and the urgency for a back up fighter reduced.

The M.20 project showcased how effective solutions could be achieved quickly by rethinking conventional approaches to design and manufacturing, particularly in a time of crisis, but the RAF no longer had an interest in the aircraft.

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In 1941 the M.20 had another lease at life, this time with the Fleet Air Arm. They were looking for a disposable fighter that could take off from ships and then ditch in the sea on return, where the pilot would be picked up.

M.20/4 fighter.
The M.20/4 version that was tested as a “disposable” fighter.

With its good performance and cheap design, the M.20 was the perfect machine for this. Miles made some changes, including adding arrestor gear and lightening components, and began testing the aircraft, now known as the M.20/4.

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It was quickly found that the fixed landing gear created a risk when ditching as they could catch the water too early. Trials continued until earl 1942, but the role would go to a modified Hurricane instead, and the M.20 was finally retired.