Airco DH . 4 The Airco DH . 4 was a British two - seat biplane day - bomber of the First World War . It was designed by Geoffrey de Havilland ( hence " DH " ) for Airco , and was the first British two seat light day - bomber to have an effective defensive armament . It first flew in August 1916 and entered service with the Royal Flying Corps ( RFC ) in March 1917 . The majority of DH . 4s were actually built as general purpose two - seaters in the USA , for service with the American forces in France . The DH . 4 was tried with several engines , of which the best was the 375 hp ( 280 kW ) Rolls - Royce Eagle engine . Armament and ordnance for the aircraft consisted of one 0 . 303 in ( 7 . 7 mm ) Vickers machine gun for the pilot and one 0 . 303 in ( 7 . 7 mm ) Lewis gun on a Scarff ring mounting for the observer . Two 230 lb ( 100 kg ) bombs or four 112 lb ( 51 kg ) bombs could be carried . The DH . 4 entered service on 6 March 1917 with No . 55 Squadron in France . Design and development . The DH . 4 was designed by Geoffrey de Havilland as a light two - seat day bomber powered by the new BHP engine . The prototype first flew in August 1916 , powered by a prototype BHP engine rated at 230 hp ( 170 kW ) . While the DH . 4 trials were promising , the BHP engine required major redesign before entering production , and the Rolls - Royce Eagle engine was selected as the DH . 4 ' s powerplant . The first order for 50 DH . 4s , powered by 250 hp ( 186 kW ) Eagle III engines was placed at the end of 1916 . The aircraft was a conventional tractor two bay biplane of all wooden construction . The crew of two were accommodated in widely spaced cockpits , separated by the fuel tank . It was armed with a single forward - firing synchronised Vickers machine gun and one or two . 303 in ( 7 . 7 mm ) Lewis Gun s fitted on a Scarff ring fired by the observer . A bomb load of 460 lb ( 210 kg ) could be fitted to external racks . While the crew arrangement gave good fields of view for the pilot and observer , it caused communication problems between the two crew members , particularly in combat , where the speaking tube linking the cockpits was of limited use . As production continued , DH . 4s were fitted with Eagle engines of increasing power , settling on the 375 hp ( 280 kW ) Eagle VIII , which powered the majority of front line DH . 4s by the end of 1917 . Because of the chronic shortage of Rolls - Royce aero engines in general , and Eagles in particular , alternative engines were also investigated , with the BHP ( 230 hp / 170 kW ) , the Royal Aircraft Factory RAF3A ( 200 hp / 150 kW ) , the Siddeley Puma ( 230 hp / 170 kW ) and the 260 hp ( 190 kW ) Fiat , all being used in production aircraft . None of these engines could match the Rolls - Royce Eagle , however there were simply not enough Eagles available . In American production , the new Liberty engine proved suitable as a DH . 4 powerplant although the engine produced a slightly inferior performance to the Eagle . The Liberty was also to eventually power the British DH . 9A . Production . Production was by Airco , F . W . Berwick and Co , Glendower Aircraft Company , Palladium Autocars , Vulcan Motor and Engineering , and the Westland Aircraft Works in the UK . A total of 1 , 449 aircraft ( from orders for 1 , 700 aircraft ) were made in the UK for the RFC and RNAS . SABCA of Belgium made a further 15 in 1926 . In the United States , the Boeing Airplane Corporation , Dayton - Wright Airplane Company , the Fisher Body Corporation , and the Standard Aircraft Corporation produced the DH - 4 with the Liberty L - 12 engine for the American air services . A total of 9 , 500 DH - 4s were ordered from American manufacturers , of which 1 , 885 actually reached France during the war . After the war , a number of firms , most significantly Boeing , were contracted by the U . S . Army to re - manufacture surplus DH - 4s to DH - 4B standard . Known by Boeing as the Model 16 , deliveries of 111 aircraft from this manufacturer took place between March and July 1920 , with 50 of them returned for further refurbishments three years later . In 1923 , the Army ordered a new DH - 4 variant from Boeing , distinguished by a fuselage of fabric - covered steel tube in place of the original plywood structure . These three prototypes were designated DH - 4M - 1 ( M for modernized ) and were ordered into production alongside the generally similar DH - 4M - 2 developed by Atlantic Aircraft . A total of 22 of the 163 DH - 4M - 1s were converted by the Army into dual - control trainers ( DH - 4M - 1T ) and a few more into target tugs ( DH - 4M - 1K ) . Thirty of the aircraft ordered by the Army were diverted to the Navy for Marine Corps use , these designated O2B - 1 for the base model , and O2B - 2 for aircraft equipped for night and cross - country flying . Operational history . British military service . The DH . 4 entered service with the RFC in January 1917 , first being used by No . 55 Squadron . More squadrons were equipped with the type to increase the bombing capacity of the RFC , with two squadrons re - equipping in May , and a total of six squadrons by the end of the year . As well as the RFC , the RNAS also used the DH . 4 , both over France and over Italy and the Aegean front . The DH . 4 was also used for coastal patrols by the RNAS , one , crewed by the pilot Major Egbert Cadbury and Captain Robert Leckie ( later Air Vice - Marshal ) as gunner , shot down Zeppelin " L70 " on 5 August 1918 . Four RNAS DH . 4s were credited with sinking the German U - boat " UB 12 " on 19 August 1918 . The DH . 4 proved a huge success and was often considered the best single - engined bomber of the First World War . Even when fully loaded with bombs , with its reliability and impressive performance , the type proved highly popular with its crews . The Airco DH . 4 was easy to fly , and especially when fitted with the Rolls - Royce Eagle engine , its speed and altitude performance gave it a good deal of invulnerability to German fighter interception , so that the D . H . 4 often did not require a fighter escort on missions - a concept furthered by de Havilland in the later Mosquito in the Second World War . A drawback of the design was the distance between pilot and observer , as they were separated by the large main fuel tank . This made communication between the crew members difficult , especially in combat with enemy fighters . There was also some controversy ( especially in American service ) that this placement of the fuel tank was inherently unsafe . In fact , most contemporary aircraft were prone to catching fire in the air . The fire hazard was reduced , however , when the pressurised fuel system was replaced by one using wind driven fuel pumps late in 1917 , although this was not initially adopted by American built aircraft . The otherwise inferior D . H . 9 brought the pilot and observer closer together by placing the fuel tank in the usual place , between the pilot and the engine . Despite its success , numbers in service with the RFC actually started to decline from spring 1918 , mainly due to a shortage of engines , and production switched to the D . H . 9 , which turned out to be disappointing , being inferior to the D . H . 4 in most respects . It was left to the further developed D . H . 9A , with the American Liberty engine , to satisfactorily replace the D . H . 4 . When the Independent Air Force was set up in June 1918 to carry out strategic bombing of targets in Germany , the DH . 4s of 55 Squadron formed part of it , being used for daylight attacks . 55 Squadron developed tactics of flying in wedge formations , bombing on the leader ' s command and with the massed defensive fire of the formation deterring attacks by enemy fighters . Despite heavy losses , 55 Squadron continued in operation , the only one of the day bombing squadrons in the Independent Force which did not have to temporarily stand down owing to aircrew losses . After the Armistice , the RAF formed No . 2 Communication Squadron , equipped with DH . 4s to carry important passengers to and from the Paris Peace Conference . Several of the DH . 4s used for this purpose were modified with an enclosed cabin for two passengers at the request of Andrew Bonar Law . These aircraft were designated DH . 4A , with at least seven being converted for the RAF , and a further nine for civil use . United States military service . At the time of its entry into the war , the United States Army Air Service lacked any aircraft suitable for front line combat . They therefore procured various aircraft from the British and French , one being the DH . 4 . As the DH - 4 , it was manufactured mostly by Dayton - Wright and Fisher Body for service with the United States from 1918 , the first American built DH - 4 being delivered to France in May 1918 , with combat operations commencing in August 1918 . The powerplant was a Liberty L - 12 of 400 hp ( 300 kW ) and it was fitted with two . 30 in ( 7 . 62 mm ) Marlin machine guns in the nose and two . 30 in ( 7 . 62 mm ) Lewis gun s in the rear and could carry 322 lb ( 146 kg ) of bombs . it could also be equipped with various radios like the SCR - 68 for artillery spotting missions . The heavier engine reduced performance compared with the Rolls - Royce powered version , but as the " Liberty Plane " it became the US Army Air Service standard general purpose two - seater , and on the whole was fairly popular with its crews . Aircrew operating the DH - 4 were awarded four of the six Medals of Honor awarded to American aviators , with First Lieutenant Harold Ernest Goettler and Second Lieutenant Erwin R . Bleckley receiving posthumous awards after being killed on 12 October 1918 attempting to drop supplies to the Lost Battalion of the 77th Division , cut off by German troops during the Meuse - Argonne Offensive , while Second Lieutenant Ralph Talbot and Gunnery Sergeant Robert G . Robinson of the US Marines were awarded the Medal of Honor for beating off attacks from 12 German fighters during a bombing raid over Belgium on 8 October 1918 . The type flew with 13 U . S . squadrons by the end of 1918 . Following the end of the First World War , America had a large surplus of DH - 4s , with the improved DH - 4B becoming available , although none had been shipped to France . It was therefore decided that there was no point in returning aircraft across the Atlantic , so those remaining in France , together with other obsolete observation and trainer aircraft , were burned in what became known as the " Billion Dollar Bonfire " . With limited funds available to develop and purchase replacements , the remaining DH - 4s formed a major part of American air strength for several years , using them for many roles , with as many as 60 variants produced . DH - 4s were also widely used for experimental flying , being used as engine testbeds and fitted with new wings . They were used for the first trials of air - to - air refueling on 25 June 1923 , and carried out an endurance flight of 37 hours , 15 minutes on 27 – 28 August , being refueled 16 times and setting 16 new world records for distance , speed and duration . The DH - 4 remained in service with the United States Army until 1932 . DH - 4s were also used by the United States Navy and Marine Corps , both during the First World War and postwar . The Navy and Marines received 51 DH - 4s during the First World War , followed by 172 DH - 4B and DH - 4B - 1 aircraft postwar and 30 DH - 4M - 1s with welded steel - tube fuselages ( redesignated O2B ) in 1925 . They remained in service with the Marines until 1929 , being used against rebel factions in Nicaragua in 1927 , carrying out the first dive - bombing attacks made by U . S . military forces . The U . S . Navy converted some DH - 4M - 1s into primitive air ambulances which could carry one stretcher causality in an enclosed area behind the pilot . Civil use . Following the end of the First World War , DH . 4s and 4As were used to operate scheduled passenger services in Europe by such airlines as Aircraft Transport and Travel , Handley Page Transport and the Belgium airline SNETA , " G - EAJC " of Aircraft Transport and Travel flying the first British commercial passenger service from Hounslow Heath Aerodrome to Paris Le Bourget on 25 August 1919 , carrying a reporter from the " Evening Standard " newspaper and a load of newspapers and other freight . They were used by Aircraft Transport and Travel until it shut down in 1920 , while Handley Page Transport and SNETA continued operating the DH . 4 until 1921 . One aircraft was used by Instone Air Line s until its merger into Imperial Airways in 1924 . DH . 4s were also used by the Australian airline QANTAS , flying its first airmail service in 1922 . Twelve DH . 4s forming part of the Imperial Gift to Canada were used for forestry patrol and survey work , spotting hundreds of forest fires and helping to save millions of dollars worth of timber , with the last example finally being withdrawn in 1927 . The U . S . Post Office also adopted the DH - 4 to carry airmail . In 1919 , the DH - 4B was standardised by the US Post Office , being modified to be flown from the rear cockpit with a 400 lb ( 180 kg ) watertight mail compartment replacing the forward cockpit . The airmail DH - 4B were later modified with revised landing gear and an enlarged rudder . DH - 4s were used to establish a coast - to - coast , transcontinental airmail service , between San Francisco and New York , a distance of 2 , 680 mi ( 4 , 310 km ) , involving night flight , the first services starting on 21 August 1924 . The DH - 4 continued in Post Office service until 1927 , when the last airmail routes were passed to private contractors .