At the time of the outbreak of World War II a wooden aircraft design was theoretically an anachronism, yet the DH.98 design would soon establish itself at the forefront of military aviation. De Havilland had a long experience with advanced wood construction techniques as applied in the DH.88 Comet and DH.91 Albatross. During the summer of 1938 De Havilland had started a study to develop the Albatross into a high speed bomber, The design, smaller than the Albatross in both fuselage and wing and to be powered by two Rolls Royce Merlin engines, would be able to carry a bomb load of 6,000lb to Berlin at such a high speed, that there was no need for any defensive armament. It was only after start of the German invasion of Poland in September 1939, that the design was subjected to official investigation by the Air Ministry. Although there was much debate on the need of self-protection for the new aircraft, on 29 December 1939, De Havilland was finally authorised to proceed with the development of the unarmed fast bomber design. At that time there was an equal interest in the Air Ministry for a photographic reconnaissance version.
Design work under the direction of R.E. Bishop, De Havilland's Chief Designer, gained pace and on 1 March 1940 De Havillands were awarded a contract for 50 bomber aircraft of the DH.98, the Mosquito.

The first prototype, serial W4050, was built in a small hangar near the DH design office at Salisbury Hall. After completion, it was transported to Hatfield and on 25 November 1940, made its first flight, only eleven months from the start of the design work, piloted by Geoffrey de Havilland Junior. Another two prototypes were constructed: W4051, the PR.I prototype and W4052 the F.II prototype. W4051 made her first flight on 10 June 1941 and was handed over to the Photographic Reconnaissance Unit at RAF Wick, Scotland, the following month.


Mosquito NF.II
The Mosquito F.II prototype, W4052, made her first flight on 15 May 1941, from a field near Salisbury Hall, Colney. In June the new machine went to the Aeroplane & Armament Experimental Establishment at Boscombe Down for handling trials.
W4052 was fitted with an Airborne Interception (A.I.) Mk.IV radar set, the arrowhead-shaped antenna of the radar transmitter mounted on the nose, just above the four Browning machine guns, whilst the dipole antennas for the receivers were mounted on the wings, near the wing tips, with two antennas on the wing and two under the wing. From early 1941, the A.I. Mk.IV radar had been in operational service in the Bristol Beaufighter. This radar operated at a frequency of about 193 MHz with a wavelength of 1.5 metres. In the cockpit two cathode ray tubes were installed for the navigator/radar operator. A target aircraft could be detected over a range of at least three miles, provided this target was about three miles above the ground. The minimum range was about 400 feet, in many situations too far for a visual contact, especially in adverse weather conditions.
Later NF.IIs were fitted with the A.I Mk.V radar, which was generally similar to the A.I. Mk.IV but included a cockpit panel display for the pilot that helped the radar operator direct him to a target. The radar equipment on the Mosquito was classified as top secret and NF.IIs carrying it were not allowed to fly over occupied Europe for some time.

One early NF.II, W4087, was fitted with a "Turbinlite" searchlight in the nose. This had 2,600 million candlepower from 24 batteries stowed behind the four cannon. The Turbinlite, developed by General Electric Co, was intended to supplement the capacity of the early A.I. radars, to ensure a visual contact . However trials showed the scheme to be ineffective. A few F.IIs (W4073, W4075 and W4077) were built with dual controls, becoming the basis for the "Trainer Mark III", most of which were unarmed, except for some that retained their guns for weapons training. The first T.IIIs were built in 1943, though most production of this variant would take place after the war, with a total of 358 built, including six conversions from F.IIs.


Mosquito NF.II into service
The first operational night fighter units became No.157 Squadron at RAF Castle Camps in January 1942 an No.151 Squadron at RAF Wittering (with a detachment at Coltishall in April 1942).
No.23 Squadron at RAF Ford on England's south coast followed in June 1942. This squadron employed some twenty-five Mosquitos in the long-range (night) intruder role over occupied northwestern Europe. These Mosquitos had the A.I. radar removed, but had, to ease navigation over enemy territory, 'Gee' equipment installed. Also an additional 150 Imperial Gallon (682 litres) of extra tankage was carried. These aircraft were designated NF.II (Special). From December 1942, the long range intruders of No.23 Squadron moved to RAF Luqa, Malta.



Mosquito NF.II production
Altogether 495 F.II/NF.IIs (including the two F.II prototypes W4052 and W4053) were built, with production split between the De Havilland works at Hatfield (366 aircraft) and Leavesden (129 aircraft). This number also includes NF.II DD715, converted to NF.XII prototype and 95 aircraft from 3 batches (HK107-HK141; HK159-HK204 and HK222-HK235). These aircraft were built as NF.II at Leavesden, but delivered to Marshalls of Cambridge for conversion to Mosquito NF.XII. This mark was equipped with the much improved A.I. Mk.VIII radar.


Preserved Mosquito NF.II
There are at the moment no airworthy Mosquito NF.IIs, but on display at the Lincolnshire Aviation Heritage Centre at the former RAF East Kirkby is HJ711. Since he bought the remains of this aircraft in April 1972, owner Tony Agar has been working on the restoration of this night fighter, "The Spirit of Val".
HJ711 was built at Hatfield and completed in May 1943. First allocated to No.60 Operational Training Unit, HJ711's first operational unit became No.141 Squadron in October of that year, before moving to No.169 Squadron ar RAF Little Snoring. HJ711 is painted in the markings of this unit and has her wartime code 'VI-P' applied. In the night of 30/31 January 1944 Squadron Leader J.A.H. 'Joe' Cooper and navigator Flight Lieutenant Ralf. D. Connely scored 169's first victory, when the shot down a Bf 110 night fighter of NJG 3 in the Berlin area.

In the restoration of HJ711 parts of PF468, PF498, VA878, RS715 have been used. After spending at the Yorkshire Air Museum at Elvington, HJ711 was taken to East Kirkby in August 2017. With the restoration work more or less completed, the port Merlin 25 fired up for the first time in August 2019.