Retrospectively allocated the company designation H.P.12 when Handley Page introduced this system in 1924, the Type O/400 was a logical development of the Type O/100 with the numerical suffix in its designation now indicating engine horsepower rather than the span of 100 ft 0 in (30.48 m). The improved bomber was intended mainly for service with the Royal Flying Corps, which had followed the lead of the Royal Naval Air Service into the heavy bombing role after it realised the significance of the German ‘Türkenkreuz’ (Turk’s cross) bomber offensive against the UK that started in February 1917 and began to achieve significant morale effect in the late spring and summer of that year. Up to that time the RFC had believed in the greater importance of the light day bomber, which offered speed rather than bomb load as its major tactical attribute, in the belief that a smaller weight of bombs delivered more accurately by day was of greater value than a larger weight of bombs delivered less accurately by night. So strong was this belief in the minds of senior RFC commanders that as late as July 1917 they decided to end all development of heavy bombers.
By August of the same year the RFC had been forced to reconsider its position in the light of the successes achieved by the Germans’ Gotha bombers and revelations about the accurate bombing achieved at night by the RNAS’s small Type O/100 force. A first step in the re-adoption of the heavy bomber concept for the RFC was the August 1917 placement of orders for prototypes of the Handley Page Type V/1500 heavy and Vickers F.B.27 Vimy medium night bombers, and then the contract for 100 initial examples of the Type O/400 improved version of the Type O/100.
The principal difference between the Type O/400 and its Type O/100 was the latter’s powerplant of two Rolls-Royce Eagle VIII water-cooled V-12 engines in place of the former’s pair Eagle II and IV engines. Final clearance for the use of the Eagle VIII powerplant was delayed, though, by the time it took to complete reports on the fuel system, and because Rolls-Royce was unable to meet delivery schedules with both left- and right-handed versions of the new engine, still assumed to be essential in large twin-engine aircraft to obviate control asymmetry problems caused by engine torque. The reasons for and process leading to the introduction of the Type O/400 had evolved throughout much of 1917. It is true that the arrival in service of the Type O/100 had marked the beginning of a major enhancement of the RNAS’s strategic striking power of the RNAS. Some 46 Type O/100 bombers had been ordered. These had given good service, but by the standards of mid-1917 were becoming deficient in performance, and their use of left- and right-handed engines made engine maintenance and replacement more complicated. Thus a standard Type O/100 was allocated in the summer of 1917 for the development of an improved bomber. This was the period in which German bombers began their short campaign of daylight attacks on south-east England and London. This prompted the British decision decision to expand the RFC by the creation of many new light bomber squadrons. Then in September of the same year the Germans switched to night raids in a programme that had little in the way of real militarily import but served to focus the Air Board on the desirability of boosting the British bombing capability against German towns and cities much farther behind the Western Front. In October Major General Hugh Trenchard, commanding the RFC in France, was ordered to create a dedicated bombing force, and established the 41st Wing with this role in mind. With the number of Type O/100 bombers now declining, there was increasing pressure to introduce the Type O/400 into production as speedily as possible.
The set-aside O/100 was test flown first with Rolls-Royce Eagle IV engines rated at 320 hp (239 kW), and then with Sunbeam Maori water-cooled V-12 engines each rated at 275 hp (205 kW), and after the success of these trials an order for 100 Type O/400 bombers was placed with Handley Page. The company could not tool up quickly to cope with this production, however, so Handley Page-built components were immediately despatched to the Royal Aircraft Factory at Farnborough, where the first 12 aircraft were hand-built. It then became clear that the assumed benefit of handed engines was wrong (and had indeed been the cause of directional instability in the Type O/100) and that the torque effects of two identical engines and propellers could be overcome by adjusting the incidence angle of the central fin of the Type O/400. The exposure of the fallacious nature of the belief in the need for handed engines immediately ended Rolls-Royce’s production difficulties and enabled the delivery schedule of single-type Eagle VIII engines to be met. Even so, a three-month delay had resulted.
The validity of this British decision was forcibly borne home from the beginning of the following month when the Germans switched their bomber effort from day to night attacks on English targets: the raids were successful, and German losses declined. As a result, the Air Board ordered another 200 Type O/400 bombers and then a further 100 in response to the request of Field Marshal Sir Douglas Haig, commanding the British Expeditionary Force in France, that 25% of all new bombers for the Western Front should be completed as night bombers.
As noted above, the development of the Type O/400 was the direct result of operational experience with the Type O/100 by the RNAS in France, and the two most important changes involved in turning the Type O/100 into the Type O/400 were the powerplant of two Rolls-Royce Eagle VIII same-handed engines in place of the Type O/100’s pair of Eagle II and IV opposite-handed engines, and the adoption of a totally revised fuel system: whereas the Type O/100 carried two 120 Imp gal (144.1 US gal; 545.5 litre) tanks in the engine nacelles to supplement the 130 Imp gal (156.1 US gal; 591 litre) tank in the central fuselage, the Type O/400 had two 130 Imp gal (156.1 US gal; 591 litres) tanks in the central fuselage, each of these being surmounted by a wind-driven pump that drove the fuel directly to the engine carburettors, and also into two 14 Imp gal (16.8 US gal; 63.6 litre) gravity-feed tanks in the upper-wing centre section, which could supply the engines in the event of a pump failure. This change allowed the shortening and therefore lightening of the two engine nacelles, which still carried the oil tanks for each engine and were supported by a different arrangement of struts, and the only other visible changes were the adoption of a full-length interplane strut behind each engine nacelle and the movement of the central fin farther to the rear so that its leading edge was slightly behind those of the fixed tailplanes. The powerplant which was intended for the Type O/400 was a pair of Rolls-Royce water-cooled V-12 engines in their most recent and uprated forms, namely the Eagle IV and Eagle VIII rated at 284 and 360 hp (212 and 268.5 kW) respectively, but production of the Eagle was never adequate to the demands placed upon it, and a number of other engines was evaluated: these included the Fiat A.12bis water-cooled six-cylinder inline engine rated at 260 hp (194 kW) and the Maori mentioned above.
The Type O/400 inherited the basic structure of the Type O/100 and was therefore of perfectly conventional design and construction for its time with a covering of fabric over a wire-braced structure of wood. The fuselage was of rectangular section and produced in three parts of which the primary section was the central unit including the internal weapons bay with spring-loaded doors, and to this core were attached the nose and tail sections. The tail unit comprised biplane horizontal and triple vertical surfaces. The horizontal surfaces each included a fixed tailplane, of which the lower unit was attached to the lower longerons, and horn-balanced elevator halves: the upper tailplane was supported above the lower tailplane by an arrangement of four vertical struts flanking the central fin above the rear fuselage and two more vertical struts forward of the pair of balanced horn-rudders.
The unstaggered wing cellule was of the three-bay type with a lower wing that was of 30 ft 0 in (9.14 m) lesser span than that the upper wing. The upper and lower wings were each of constant thickness and chord, but while the lower wing was built in four sections, the longer-span upper surface was produced in five sections. The lower wing was based on flat centre-section halves that extended from the lower longerons and carried the two outer panels, which were each slightly dihedralled and ended in rounded tips. The upper wing was based on a flat centre section carried above the fuselage by fore-and-aft sets of inverted-V struts, and this carried the two slightly dihedralled outer wing panels that were each built in two sections with the outer section, braced by an overhead kingpost-and-cable arrangement, ending in a square-cut tip whose forward edge was rounded off and whose trailing edge was occupied by the horn-large balanced aileron. The outer wing panels were separated by two sets of parallel interplane struts and could be folded to the rear to reduce hangarage requirements, and the whole wing cellule was braced with the normal arrangement of flying and landing wires.
The airframe was completed by the landing gear, which was of the fixed tailskid type. Each main unit comprised a two-wheel sprung axle carried at the base of two strut-braced N-type strut arrangements extending vertically below the inboard and outboard ends of the lower-wing centre section.. The powerplant of two Eagle engines, each driving a four-blade wooden propeller and cooled by a large frontal radiator, was installed in a pair of short nacelles carried in the interplane gap between the outer ends of the upper- and lower-wing centre sections: each nacelle was supported above the lower wing on a fore-and-aft pair of short V-type struts, and itself supported the outer end of the upper-wing centre section with a fore-and-aft pair of inverted V-type struts.
Production of the Type O/400 was 554 aircraft out of an ordered 788 aircraft completed by Handley Page (211 aircraft out of an order for 324 of which 113 were later cancelled), Birmingham Carriage Co. (102 aircraft out of an order for 120 of which 18 were later cancelled), Royal Aircraft Factory (24 aircraft), Metropolitan Waggon Co. Ltd. (100 aircraft out of an order for 175 of which 75 were later cancelled), Clayton & Shuttleworth Ltd. (46 aircraft out of an order for 50 of which four were later cancelled), National Aircraft Factory No. 1 operated by Cubitt Ltd. (70 aircraft produced in component form in the USA by Standard Aircraft out of an order for 100 of which 30 were later cancelled), and Harland & Wolff Ltd. (one aeroplane as replacement for the pattern machine diverted to the Standard Aircraft Corporation in the USA).
In the event the first hand-built Type O/400 bombers from Farnborough were completed only a few weeks before the first aircraft from Handley Page’s Cricklewood factory, and it was in April 1918 that the first aircraft reached the squadrons of what had been, since 1 April, the new Royal Air Force combining the previously separate RFC and RNAS. In April Nos 207 and 215 Squadrons took delivery of their full complements at Netheravon, the former using Farnborough-built aircraft and the latter Cricklewood-built aircraft. At almost the same time No. 216 Squadron took delivery of its first aircraft, replacing the Royal Aircraft Factory F.E.2b, at Cramaille in France. As these squadrons began preparing for combat, the 41st Wing had increased in size to become the VIII Brigade, and on 6 June the Independent Force officially came into existence with, by the end of August, four squadrons with the Airco (de Havilland) D.H.9 and newer D.H.9a day bomber, and four (Nos 97, 115, 215 and 216 Squadrons) with the Type O/400 night bomber. Elsewhere in France, Nos 100, 207 and 214 Squadrons also operated the Type O/400 on a temporary basis but separate from the Independent Force.
It was in the middle of August 1918 that the Independent Force received its first complete Type O/400 unit when No. 97 Squadron arrived in France, this being followed later in the same month by No. 215 Squadron from England and No. 100 Squadron that converted in France. In France, the Type O/400 eventually equipped Nos 58, 207 and 214 Squadrons of the IX Brigade’s 82nd, 54th and 82nd Wings respectively as well as Nos 97, 100, 115, 215 and 216 Squadrons of the 83rd Wing of the Independent Force’s VIII Brigade. The new bomber proved very successful, though their achievements were not achieved without loss, for between June and November 1918 the Independent Force lost 69 of the aircraft, 18 of them on operations and the other 51 in accidents. In Palestine one O/400 was attached to No. 1 Squadron of the Australian Flying Corps, and numbers of Type O/400 warplanes were also used in the UK by No. 1 and No. 2 Schools of Air Navigation and Bomb-dropping based at Stonehenge and Andover respectively.
Though the airframe of the Type O/400 differed but little from that of the Type O/100 and the internal bomb load was the same, the Type O/400’s greater power and reduced fuel consumption enabled it to carry heavier bomb loads without sacrifice of any of the fuel. The increased power of the engines and reduced drag of the nacelles with their associated mounting struts resulted an increase of 12% in cruising speed which, with the endurance remaining at about eight hours, resulted in a range increase of some 100 miles (160 km) with the same bomb load. Production of the 520-lb (236-kg) light case and 550-lb (249-kg) heavy case bombs had increased by 500% during 1917 and, when carrying three of these bombs internally, the Type O/400 could also lift two 112-lb (51-kg) bombs on external racks under the fuselage, and still carry full fuel. Another bomb tested in 1917 was the 1,650-lb (748-kg) SN, but this heavy cast weapon did not become available until July 1918; an improved version, the 1,800-lb (816-kg) SN (Mod), specially tailored to the Type O/400 and like the standard SN carried under the fuselage, became available in August 1918.
After the end of World War I the RAF decided to standardise on the Airco (de Havilland) D.H.10 Amiens and Vickers F.B.27 Vimy bombers, and the number of Type O/400 bombers declined rapidly, the last two units being Nos 70 and 216 Squadrons that flew the type from Egyptian bases up to 1920.
The Type O/400 was also one of the British types selected by the USA during 1917 for large-scale American manufacture, and contracts were placed with the Standard Aircraft Corporation for 1,500 aircraft including large numbers completed only in component form for assembly in the UK and/or France. The American variant of the Type O/400 had a powerplant of two Liberty 12N water-cooled V-12 engines each rated at 360 hp (268.5 kW) and could carry a bomb load of 3,000 lb (1361 kg), and this variant otherwise differed from the standard British-built O/400 in details such as its empty weight of 7,894 lb (3581 kg), maximum take-off weight of 14,300 lb (6486 kg), maximum speed of 80 kt (92 mph; 148 km/h) at sea level declining to 74 kt (85 mph; 137 km/h) at 10,000 ft (3050 m), climb to 5,000 ft (1525 m) in 12 minute 0 seconds, and service ceiling of 10,000 ft (3050 m). The first American-built Type O/400 bomber flew in July 1918, and deliveries of complete aircraft and kits of components had reached only 107 before the end of World War I and the cancellation of the balance of the orders. Only a small number of kits were assembled in the UK, and by the end of June 1919 the type had virtually disappeared from even limited American service.
After the war surplus aircraft were converted for civilian use in the UK, and nine of these were used by Handley Page’s pioneering airline, Handley Page Transport. An initial six (later 12) Type O/7 aircraft were assembled after the war for the Republic of China, for use principally as 14-passenger transports. These were delivered to China and re-assembled at Nanyuan near Beijing. The aircraft flew their first service, carrying both air mail and passengers, between Beijing and Tientsin on 7 May 1920. The civil services were disrupted by the outbreak of civil war, with the aircraft being taken over by various warlords. During the 1st Zhili-Fengtian War (1922), O/7 bombers carried three 200-lb (91-kg) bombs, and played a major part in the victory of the Zhili clique as the opposing Fengtian faction had only liaison and reconnaissance aircraft. During the 2nd Zhili-Fengtian War (1924), O/7 bombers of the Fengtian faction carried a single 500-lb (227-kg) bomb and played an important role in the Battle of Stone Gate Camp near Shanhai Pass on 19 October. An O/7 bomber, flown by a White Russian, dropped its single 500-lb (227-kg) bomb on the densely packed Zhili force on the ground, causing large casualties, which in turn led to the collapse of the Zhili morale, resulting in a Fengtian victory.
Poland bought one Type O/400 in 1920.
Handley Page Type O/400
Type: heavy night bomber
Accommodation: bomb-aimer/gunner in the open nose position, pilot and co-pilot or optional observer/navigator side-by-side in the open cockpit, gunner in the open dorsal position, and provision for gunner in the open ventral position
Powerplant: two Rolls-Royce Eagle VIII liquid-cooled V-12 piston engines each rated at 360 hp (268.5 kW) for take-off
Performance: maximum speed 85 kt (97.5 mph; 157 km/h) at sea level declining to 75.5 kt (87 mph; 140 km/h) at 5,000 ft (1525 m) and to 69.5 kt (80 mph; 129 km/h) at 10,000 ft (3050 m); climb to 5,000 ft (1525 m) in 23 minutes 0 seconds and to 6,500 ft (1980 m) in 27 minutes 10 seconds; service ceiling 8,500 ft (2590 m); endurance 8 hours 0 minutes
Weights: empty 8,502 lb (3857 kg); maximum take-off 13,360 lb (6060 kg) with 16 112-lb (51-kg) bombs
Dimensions: span 100 ft 0 in (30.48 m); area 1,648.00 sq ft (153.10 m²); length 62 ft 10.25 in (19.16 m); height 22 ft 0 in (6.71 m); tailplane span 16 ft 7.5 in (5.07 m); wheel track, each unit 4 ft 6 in (1.37 m)
Armament: one or two 0.303-in (7.7-mm) Lewis trainable forward-firing machine guns in the nose position, one or two 0.303-in (7.7-mm) Lewis trainable rearward-firing machine guns in the dorsal position, and one 0.303-in (7.7-mm) Lewis trainable rearward-firing machine gun in the ventral trapdoor position with a total of 1,649 rounds of ammunition for all these guns, and up to 2,000 lb (907 kg) of disposable stores carried in a lower-fuselage weapons bay rated at 2,000 lb (907 kg) and on two hardpoints (both under the fuselage), and generally comprising three 550- or 520-lb (249- or 236-kg) free-fall bombs, or eight 250-lb (113-kg) free-fall bombs, or 16 112-lb (51-kg) free-fall bombs carried internally, and two 112-lb (51-kg) free-fall bombs carried externally; one 1,650-lb (748-kg) free-fall bomb was an alternative external load