The Airco D.H.4 was the Royal Flying Corps' first purpose-built day bomber, filling a role that until then had been carried out by aircraft that had been designed for other duties.
The D.H.4 was originally designed to use a 160hp Beardmore engine, but while it was under development a new engine appeared. This was the 200hp water-cooled, six-cylinder inline Beardmore-Halford-Pullinger or B.H.P. engine, designed by F. B. Halford with the support of William Beardmore and T. C. Pullinger, the head of Arrol-Johnston Ltd. of Dumfries. This new engine was roughly the same shape as the 160hp Beardmore, although it was 25cm longer, 20cm taller and 75lb heavier.
The new engine began bench tests in June 1916, at about the same time as the prototype D.H.4 was approaching completion. The aircraft was modified to take the new engine, and made its maiden flight in August 1916, with Geoffrey de Havilland at the controls.
The D.H.4 was later used to test many other engines including the 300hp Renault 12Fe, 400hp Sunbeam Matabele, 353hp Rolls-Royce G, Ricardo-Halford inverted supercharged engine and the 400hp Liberty 12.
The D.H.4 was conventional two-bay biplane, built around a fabric covered spruce and ash framework. As on a number of earlier de Havilland designs the front of the aircraft was reinforces with plywood, producing a light but strong structure. The pilot's cockpit was located under the centre section of the upper wing, below a transparent area of the wing. The D.H.4 had the same curved rudder shape as the D.H.3 and most later de Havilland aircraft.
Despite its impressive performance the original B.H.P. engine was not used in production D.H.4s. A second prototype was completed with a 250hp Rolls Royce engine, and most early Airco produced aircraft were powered by either the Rolls Royce Eagle III or Eagle IV, both providing 250hp.
Rolls Royce soon ran into production problems, and a number of alternative engines had to be used to keep up with D.H.4 production. A modified version of the B.H.P. engine was produced by J. D. Siddeley as the Siddeley Puma, but this engine also suffered from production delays and wasn't available in large numbers until 1918.
When it became clear that neither Rolls Royce nor Siddeley engines would be available Airco turned to the 200hp R.A.F.3a as an alternative. No.18 Squadron was the first to receive this version of the D.H.4, in June 1917, and it was not a great success, suffering from a series of technical problems and failures, as well as being the least powerful engine used with the D.H.4.
Yet another engine appeared later in 1917. Britain had agreed to provide 50 D.H.4s to the Russian government, to be powered by 260hp Fiat A-12 engines. When the German bombing offensive forced the British to plan for their own offensive over Germany Russia agreed to let the R.F.C. keep these aircraft on the understanding that they would be replaced in the spring of 1918.
Finally, in the summer of 1917, a suitable engine was found in the shape of the 375hp Rolls Royce Eagle VIII and by 1918 most D.H.4s in British service used this engine, which gave the aircraft a top speed of 143mph, 35mph better than Puma powered aircraft.
A total of 1,700 D.H.4s were ordered from Airco and six main sub-contractors, of which 1,449 were eventually delivered. The D.H.4 was also produced in very large numbers in the United States, where it was powered by the 400hp Liberty 12 engine.
The D.H.4 was a popular aircraft with its pilots, who appreciated its handling, its high speed and its high rate of climb, which made it very difficult for German fighters to catch it. It did have some limits – the two cockpits were too far apart, making it hard for the pilot and observer/ gunner to communicate, and the pressurised fuel system used until late in 1917 was very vulnerable to enemy fire. The risk of fire was reduced late in 1917 when a new system using two wind-driven pumps was installed.
Combat – R.F.C./ R.A.F.
No.55 was the first R.F.C. Squadron to receive the D.H.4 when its aircraft were flown out to France on 6 March 1917. The first operation came a month later, on 6 April 1917 at the start of the battle of Arras when the squadron attacked railway sidings at Valenciennes.
No.55 Squadron was one of the few R.F.C. squadrons not to suffer heavy losses during 'Bloody April', the costly aerial fighting that accompanied the battle of Arras. While most British aircraft were outclassed by the latest generation of German fighters the D.H.4 was able to use its superior speed and climb rate to escape from attack.
Two more squadrons, Nos.18 and 57, were equipped with the D.H.4 in May, No.25 Squadron got it in June and by the end of 1917 six R.F.C. squadrons were operating the D.H.4.
Production of the D.H.4 was scaled down rather prematurely. Major-General Hugh Trenchard and other supporters of the bomber offensive believed that it was obsolete by the end of 1917, and expected it to be replaced by the D.H.9. If a powerful enough engine had been available then this would have been true, but the D.H.9 was badly underpowered and when it entered service early in 1918 the D.H.9 proved to be less capable aircraft than the D.H.4. Only when it was given the American Liberty 12 engine did the new aircraft, as the D.H.9A, finally come into its own.
As a result of this by the middle of 1918 the RAF only had nine D.H.4 Squadrons available in France, of which four were former R.N.A.S. units. Another eight squadrons were based in Britain, where they were used for home defence and training.
The only significant British variant of the D.H.4 was the D.H.4A transport aircraft. This was created during the post-war Peace Conference to allow a Minister and his secretary to talk during the flight, and contained a two-man passenger cabin.
The D.H.4 was unusual in that both the R.F.C. and R.N.A.S. acquired the aircraft at about the same time. Ninety aircraft were built by Westland to Admiralty specifications, and another sixteen were transferred to the Navy from War Office orders. The purpose built Naval aircraft differed from the standard D.H.4 in having two forward firing Vickers guns and a higher mounting for the rear Scarff ring.
No.2 (Naval) Squadron at St Pol was the first to receive the D.H.4, in March-April 1917. This was a reconnaissance squadron that operated in support of naval monitors bombarding German coastal positions. On 1 April 1918 it became No.202 Squadron, R.A.F., and in this new role played an important role in the preparation for the raid on Zeebrugge of 22-23 April 1918, taking a complete set of photographs of the port.
No.5 (Naval) Squadron was next, receiving its D.H.4s at Coudekerque in late April. This squadron operated as a conventional day bomber squadron, attacking German naval targets and air fields in Belgium.
Eventually five R.N.A.S. squadrons operated the D.H.4 in Belgium and three from British Coastal Air Stations. On 5 August 1918 a D.H.4 from RNAS Coastal Station Great Yarmouth attacked and shot down a Zeppelin, variously identified as L.44 or L.70.
The Navy also operated the D.H.4 in the Aegean (four squadrons) and Italy (three squadrons) where they operated over the Balkans, as well as attacking the German cruiser Goeben.
The D.H.4 didn't remain in service for very long after the First World War. A large number of the improved D.H.9A were built towards the end of the war, and these superseded the older bomber in the post-war RAF, while the earlier D.H.9 was more popular with overseas governments. A small number of D.H.4s were distributed as Imperial Gifts, with Canada receiving twelve, New Zealand two, and South Africa and Australia both getting small amounts. The D.H.4 was also used by Belgium, Chile, Greece, Iran and Spanin in small numbers, with some remaining in service until the early 1930s.
Engine: See above
Power: See above
Wing span: 42ft 5 5/8in
Length: 30ft 8in
Height: 10ft 1in
Tare Weight: 2,197-2,387lb
All-up Weight: 3,313-3,472lb
Max Speed: 106-143mph
Service Ceiling: 16,000-22,000ft
Endurance: 3.5-4.5 hours
Armament: One forward firing Vickers Gun, one flexible Lewis Gun.
Max Bomb-load: 460lb