Staff Writer (Updated: 5/5/2016):
Origins of the DH.98 was owed to development of all-wood de Havilland racing planes appearing in the mid-1930s as the designation of DH.88 "Comet". The extensive use of a wood (plywood/balsa) with stressed skin approach proved them somewhat of a revolutionary departure from the metal-skinned airframes beginning to take hold in military inventories around the globe. The Comet went on to claim the London-Melbourne Centenary Races and de Havilland then moved to produce an airliner-minded design utilizing the same wood approach, this giving rise to seven examples of the beautifully streamlined DH.91 Albatross first flying in 1937. Despite his wooden designs being consistently rejected by the British Air Ministry, de Havilland persisted when authorities sought a new medium bomber design through Specification P.13/36. However, once again, a modified form of the Albatross was rejected by the Air Council due to the focus falling on metal-skinned designs.
The Mosquito Takes Shape
Undeterred, de Havilland has his design team working on an all-new wooden concept as a private venture, an unarmed light bomber that would include a two-man crew in a smaller fuselage frame powered by two outboard engines and feature an internal bomb bay. The design was a true departure from the metal-encased, lumbering machine-gun-defended mounts the Air Ministry sought. De Havilland's design held many strong inherent qualities - it was dimensionally smaller and lightweight thanks to the wood construction and lack of complicated and heavy defensive turrets or gun stations. This also helped reduce a crew of six or eight men to just two - a pilot and navigator. The de Havilland design was also intended to fly higher than enemy air defenses could reach and faster than any intercepting enemy aircraft could hope to manage. The aircraft's layout was consistently de Havilland, a well-formed teardrop shape with the two-man cockpit fitted in the extreme forward portion of the fuselage, engines mounted in streamlined nacelles to either side of the cockpit and wings mid-mounted monoplane assemblies. The empennage was highly conventional for the period consisting of a single vertical tail fin and low-set horizontal tailplanes. The pilot and his navigator sat in a side-by-side arrangement.
With Germany on the move across both political and military fronts by 1939, much thought on the part of the British was in stemming the tide with the weapons and manpower available. As such, little interest remained for the de Havilland concept. It was with some help from de Havilland's World War 1 ally - Air Marshal Sir Wildred Freeman - who sat on the Air Council that the new de Havilland approach was greenlighted for development in December of 1939. The RAF contracted for a bomber prototype and 50 aircraft were slated for purchase to Specification B.1/40.
Inside the Mosquito
Internally, the pilot and navigator sat in a side-by-side formation, the pilot on the left with the navigator to the right. Entry/exit to the aircraft was through a hatch along the navigator's right. The cockpit was relatively well organized with throttle and engine gauges set along the left hand side within easy reach. Basic dials were spread about the uncluttered instrument panel which DH-98 pilots regarded as good to excellent. A small passageway along the right of the instrument panel allowed crew access to the nose which, in non-solid-nosed versions, featured a clear plastic windscreen for bombing. The cockpit was heated for high-altitude flying and generally regarded as more comfortable than the Bristol Beaufighter it replaced. Other production versions introduced cockpit pressurization for extreme high-altitude work.
Three Basic Forms
Developed in complete secrecy, the de Havilland endeavor was given the designation of DH.98 "Mosquito" and her powerplant of choice became the excellent Merlin series that also powered the famous Supermarine Spitfire fighters. The de Havilland wood technique was slightly modified for the Mosquito as the aircraft would have to be reinforced for the abuses of military combat. Development eventually went beyond the original bomber prototype (FB) and included a specialized photographic reconnaissance (PR) version as well as a dedicated night fighter (NF) variant along with basic fighter variations (F). Each type exhibited their own developmental "marks" followed by a number to signify key changes between each design.
Wood Over Metal
The principle use of wood was key to the success of the Mosquito, especially concerning its war time production. Metals proved a sought-after resource and, with it, those specialists that could work with such materials to help form the hulls of battleships and submarines, the airframes of modern aircraft or the shells of bombs. The Mosquito therefore, would rely on the readily available supplies of wood through the British Empire and essentially require the skilled services of woodworkers who were not as critical to the primary British war effort. Such a revolutionary, forward-thinking approach partially made the successes of the Mosquito design possible (her crews being the other part).
Mosquitoes were generally armed with 4 x 7.7mm Browning machine guns in the nose with 4 x 20mm Hispano cannons under the cockpit floor. Some versions did away with the machine guns altogether while other forms replaced the 4 x cannons with a single 57mm cannon. The internal bomb bay supported up to 500lbs of ordnance in original versions and this later ballooned to bomb load outs comparable to that of American B-17s. Bombs could also be added to hardpoints under each wing - initially 2 x 250lb types then 2 x 500lb types. Additionally, the Mosquito was eventually tested and cleared to fire underwing rockets. Navalized Mosquitoes were outfitted with a single torpedo under the fuselage. External fuel tanks ultimately worked they way into the DH.98's load out and offered extended ranges.
The Mosquito in Practice
The prototype Mosquito first took to the air on November 25th, 1940 and any remaining Air Ministry skeptics were immediately silenced for good. The DH.98 proved a thoroughbred in the air and only required slight modifications to flush out teething issues before serial production was set to begin. A fighter prototype was unveiled in May of 1941 and the DH.98 as a whole was formally introduced for service with the RAF that same year. The photographic reconnaissance variant was outfitted with three camera systems with an optional fourth (1 x F.52 20- or 30-inch daytime camera, K.17 6-inch survey and mapping camera and 1 or 2 x F.24 cameras). The night fighter variant was quickly ushered along and given sophisticated Airborne Interception (AI) radar for the task. The type also retained the 4 x 20mm cannon array while later versions dropped the 4 x machine guns. A bulletproof windscreen was added for crew protection and wing spars were reinforced for improved fighter-like qualities.
One of the key detriments to early Mosquito successes was in a limited production rate which, in turn, limited availability of the excellent aircraft for British flight groups. Mosquitoes were accepted into Fighter Command ranks in January of 1942 and were used to replace outmoded types such as Bristol Beaufighters as quantities allowed. The first combat-level sortie involving a Mosquito was on September 17th, 1941 when a photographic reconnaissance mount of No. 1 PRU was sent over the border between France and Spain. Night-fighting Mosquitoes proved the design supremely sound when, in the span of a few months in early 1943, pilots scored kills against no fewer than 17 enemies. The success of the fighter-bomber, reconnaissance and night fighter types did away with the planned dedicated fighter prototype in time - thusly leaving only three prototypes making up the family line which numbered over 7,000 examples.
Success for the DH.98 was not limited solely to its night-fighter forms for the aircraft also excelled in daytime sorties against all manner of enemy targets - be they in the air or on the ground. Reconnaissance versions could outpace most intercepting German fighters while intruding sorties numbering a few aircraft could be used for surprise raids against unsuspecting strategic positions. Mosquitoes could utilize their combination machine gun/cannon fire to strafe ground targets such as depots, supply trains, troop columns and airfields. Others went into battle with internal and external bomb loads for tactical bombing forays deep within enemy territory, on approached sometimes no higher than at treetop level. Still others were eventually outfitted with rockets and launched against enemy shipping where their rockets could be used to devastating effect against unsuspecting Axis vessels and submarines. A few forms even gave up their 4 x cannons for a single 57mm cannon to be used exclusively against enemy shipping in the ship-busting role - such was the flexibility of the airframe. Mosquitoes were also used by Bomber Command to "light the way" for accompanying heavy bombers in British night time raids through use of the "Oboe" target-marking facility - one of the more effective systems deployed during "blind bombing" in all of the war.
Night fighter forms were generally progressed by the evolution of better radar suites. Later forms saw their machine gun armament removed as the 4 x cannons proved equally effective on their own (this also proved a weight-saving measure). Crews operating in the thick of night relying heavily on their onboard technology to bring an enemy aircraft within range under complete surprise. The Mosquito's impressive 4 x cannon battery could bring down an enemy bomber in a few short, well-placed bursts at range. In time, a rear warning radar was also installed which furthered the capabilities of the night-fighting DH-98 crews by detecting pursuing enemy aircraft.