De Havilland Mosquito: Introduction and Initial Development

The de Havilland Mosquito was the most versatile aircraft of the Second World War, serving as a pure bomber, with a bomb load of 4,000lb, a fighter bomber, a night fighter and a high flying photo reconnaissance aircraft. When it first appeared it was the fastest aircraft yet to enter RAF service. All this in a wooden aircraft, developed despite severe reservations amongst the RAF hierarchy.

It is not hard to understand where these reservations came from. Current bombers were expected to be able to fight their way to their target, although British light bombers such as the Bristol Blenheim or Fairey Battle would turn out to be dangerously under-armed. In contrast, de Havilland proposed a light weight un-armed bomber, which would rely entirely on speed to get through. There was a real danger that newer enemy fighters would soon negate the speed advantage of the aircraft.

The idea of a bomber with the performance of a fighter was not new. When it had first appeared, the Bristol Blenheim had been significantly faster than any current fighter aircraft, although it was soon superseded by the first generation of monoplane fighters.

In 1936 the Air Ministry had issued specification P.13/36, for a twin engined medium bomber capable of carrying 1000lb of bombs 1000 miles at 15,000 feet at a cruising speed of 275mph. De Havilland was one of several companies to make a submission. Their aircraft could carry 4,000lb of bombs for 1,500 miles but only at 260 mph. One result of this work was that de Havilland became convinced that all current attempts to design this fast bomber were using too large a fuselage.

While their bomber proposals were meeting with failure, de Havilland was working on other projects that would play a part in the success of the Mosquito. The D.H. 88 Comet was a two engined racing plane, built from spruce and plywood. It was designed for, and won, the 1934 MacRobertson Air Race. The DH.91 Albatross was a streamlined four engined transport and passenger plane, designed in 1936, and also built from wood.

The experience de Havilland gained on these two aircraft convinced them that a small streamlined wooden twin engined aircraft could be made fast enough to evade any potential fighter opposition. Wooden construction would also have the advantage of using a non-strategic material – one that could be acquired easily by Britain – and using the skills of the large number of wooden furniture makers to be found in Britain.

De Havilland submitted two similar design proposals to the Air Ministry in September 1939. Project D.H. 98 was selected for further work, and on 12 December 1939 de Havilland were commissioned to produce a prototype. Specification B.1/40 was produced to describe the new aircraft. Things had moved on dramatically from 1936. This new specification called for a maximum speed of 397mph, and a service ceiling of 32,100 feet.

The new design was for a wooden aircraft. The 52ft 6inch wide wing was constructed in one piece from spruce and plywood. Power was provided by two Rolls Royce Merlin engines. Their radiators were placed on the wing leading edge, between the engine nacelle and the main fuselage, allowing for more streamlined engine nacelles and reducing drag. The main fuselage was built in two halves. As much work as possible was done while the two halves of the aircraft were separate. They were then joined together, and finally covered in fabric. The first prototype had a clear glass nose. Several accounts by Mosquito crews mention the navigator or bomb aimer lying on their belly in the nose of the aircraft.

On 1 March 1940 de Havilland received a production order for 50 bomber/ reconnaissance Mosquitoes (this number included the original prototype). This order was amended several times. In November 1940 it was reduced to 49 aircraft, of which 28 were to befighters, including a prototype. On 17 July 1941 the order was altered again, this time to convert nine of the reconnaissance aircraft to bombers. In the end forty nine aircraft were built to this order. Of those forty nine aircraft, three were the initial prototypes, one was the prototype B Mk V, two were constructed with gun turrets before being converted into T Mk IIIs, four were built as T IIIs, nine became B Mk IV series i bombers and only nine were completed as photo reconnaissance PR Mk Is. Later production orders tended to be a bit more focused!

Despite this early order, work progressed slowly. De Havilland were heavily involved in the production of the Tiger Moth, as well advanced variable pitched propellers needed by many other aircraft. The collapse of France and the resulting threat of German invasion led to a ban on any work that might interfere with the production of existing first line aircraft. The Mosquito came close to being cancelled on several occasions during the 1940s. On 5 October 1940, with work on the prototype nearly finished, the Mosquito design team was moved from the main de Havilland factory at Hatfield to nearby Salisbury Hall.

Despite these delays, by November 1940 the first prototype Mosquito (W4050) was ready to be moved from Salisbury Hall back to the factory. After taxi trials on 24 November, the Mosquito made its first test flight on 25 November 1940. The test pilot was Geoffrey de Havilland, Jr, son of Sir Geoffrey de Havilland, the founder of the company.

It was immediately apparent that the Mosquito lived up to every promise de Havilland had made. It was significantly faster than the current models of the Spitfire, itself a very fast aircraft. When the Mosquito entered service in 1941, the PR Mk I had a top speed of 382 mph, while the Spitfire Mk V could only reach 369 mph. On 30 December 1940 the Air Ministry placed an order for 150 more Mosquitoes. From then on the only complaint the RAF had about the Mosquito was that there were never enough of them!

The amazing versatility of the Mosquito is perhaps best demonstrated by looking at the first three aircraft. W4050 we have already seen. It was the prototype for an unarmed bomber.

Next to appear was W4052, the prototype for a Mosquito fighter. It made its first flight on 15 May 1941 and was designed to specification F.21/40. It was armed with four .303in Browning machine guns located in a new solid nose, and four 20mm cannon located under the fuselage, with part of the cannon located in the bomb bay. The crew entrance had to be moved from its original location under the cockpit to a new position on the side of the aircraft. The v-shaped windscreen of the bomber was replaced by an optically neutral flat windscreen. Two aircraft were modified to carry a gun turret, but that idea was quickly abandoned.

The third aircraft (W4051) was the photo reconnaissance prototype. It first flew on 10 June 1941, having been delayed after its fuselage was used to repair the first prototype. It had longer wings than the original prototype – 54 feet 2in wide, and could carry a wide range of cameras. W4051 eventually joined the PRU and became an operational aircraft.

Even these three prototypes did not complete the picture. In July 1941 work began on a fighter bomber Mosquito, combining the eight gun firepower of the original fighters, with a 1,000lb bomb load (later increased to 3,000lbs). By the end of the war, the Mosquito could carry the 4,000lb “cookie” bomb, was acting as an intruder over German fighter bases, had sunk U-boats, and had carried out some of the most dramatic precision bombing raids of the war. While later German fighters did eventually catch up with the Mosquito’s speed, they never gained enough of an advantage to effectively intercept it. It had even inspired a German copy, the Focke-Wulf Ta 154 “Moskito”, also a twin engined wooden aircraft.

Mosquito Aces of World War 2, Andrew Thomas. This volume concentrates on the fighter variants of the Mosquito, looking at their role as a defensive fighter, both over Britain and overseas and their use during the D-Day invasion to protect the fleet. Thomas also looks at the career of the Mosquito as a night intruder over Germany, where it became the scourge of the German night fighters, often being blamed for losses miles from the nearest Mosquito.
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How to cite this article: Rickard, J (15 April 2007), De Havilland Mosquito: Introduction and Initial Development,

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