The O/400 did not appear from nothing. In December 1914 the Admiralty issued a requirement for a large aeroplane capable of carrying a worthwhile bomb load over long distances — a machine that could reach the German airship sheds. The demand is remembered through the phrase calling for a “bloody paralyser of an aeroplane”, a line that has been attached to several senior RNAS figures over the years; the wording varies between accounts, so it is best treated as the spirit of the requirement rather than a verbatim quotation from one named officer.

Frederick Handley Page answered with the O/100, first flown in December 1915: a twin-engined biplane of unprecedented size for a British aircraft. Forty-six O/100s were built for the RNAS. Experience in service led to the improved O/400, which standardised on more powerful Rolls-Royce Eagle VIII engines and a revised fuel system. The two types are often discussed together as the “Handley Page Type O”, and in the field they served side by side.

It is worth pausing on how bold the concept was. In 1914 most military aircraft were small, unarmed scouts used for reconnaissance; the notion of a purpose-built heavy bomber able to carry a tonne of bombs deep behind the lines was close to science fiction. The O-series turned that notion into hardware, and in doing so helped define a whole category of aircraft that had not existed before.

Why it mattered

The O-series gave Britain, for the first time, an aircraft designed from the outset to deliver heavy bombs at range. Everything the Independent Air Force attempted in 1918 rested on that foundation.

Two features defined the machine. The first was its folding wings. The original naval requirement demanded that so large an aeroplane still fit inside existing hangars and sheds, so the outer wings were hinged to fold back alongside the fuselage — an elegant solution that turned a hundred-foot span into something a ground crew could store and shelter. The second was its powerplant: two Rolls-Royce Eagle VIII V-12 engines of about 360 horsepower each on the O/400, mounted in nacelles between the wings and driving tractor propellers. (The earlier O/100 used lower-powered Eagle IIs; some aircraft, including many intended for American production, were fitted with other engines such as the Liberty or Sunbeam Maori, so engine fits varied.)

The Rolls-Royce Eagle was one of the great engines of the war — a water-cooled V-12 developed from the company’s pre-war motor-car engineering — but it was also in high demand and short supply, which constrained how many bombers could be built and kept flying. Two of them per aircraft made the O/400 a thirsty, resource-hungry machine to field in quantity.

The result was an aircraft that was steady and load-carrying rather than fast or agile. Handled gently it was a fine night bomber; thrown around, its long wings and heavy structure punished the careless.

The O/400 carried its bombs internally in a cell in the fuselage, plus underwing stations, for a maximum load usually given as around 2,000 lb (roughly 900 kg). Typical fits included sixteen 112 lb bombs, eight 250 lb bombs, or three larger weapons. Late in the war the type could also carry a single 1,650 lb bomb — the heaviest bomb dropped by the RAF during the First World War — a weapon meant for hardened targets and for the shock value of a very large explosion.

The actual load carried on any given night depended on the target and the distance: fuel for a long trip to the Ruhr meant fewer bombs, while a shorter raid on the Belgian coast allowed a heavier load. Bomb aiming was done with the simple sights of the day, so accuracy was limited and crews aimed for large targets — rail yards, factories, airfields — rather than pinpoint objectives.

Defensively the bomber bristled with .303 in Lewis guns — commonly up to five — on a nose scarff ring, in a dorsal position and firing downward through a ventral hatch to guard the blind spot beneath the tail. At night, however, its real defence was darkness.

No reliable unit price or hourly operating cost survives for the O/400 in a form worth quoting. It was a wartime aircraft, procured under emergency conditions and accounted for very differently from a modern machine, and the surviving records do not translate into a meaningful “cost per flight hour”. What can be said is that the O/400 was expensive in the terms that mattered in 1918: it needed two of the scarce, costly Rolls-Royce Eagle engines, a large skilled crew, and a great deal of fuel for its long night sorties.

Because no complete O/400 survives and none is airworthy, there is also no modern operating cost and nothing to book. Any figure you see presented as a per-hour cost for this aircraft is not grounded in a real, flyable example.

The most historically important chapter in the O/400’s career is its service with the Independent Air Force in 1918. This was a force deliberately detached from the direct support of the army and navy so that it could strike targets inside Germany — industry, railways and the war economy — rather than the trenches. Its O/400 squadrons flew by night from fields in eastern France to bomb the Rhineland, the Saar and the Ruhr.

The material damage was modest by later standards, and the campaign was cut short by the Armistice. Its significance is as a precedent. Here, for the first time, was a national force organised around the idea that bombers could carry a war to an enemy’s heartland — the direct counterpart to the German Gotha and Zeppelin raids on London, and the ancestor of the strategic bombing of the Second World War. The O/400 was the aircraft that made the argument real.

When the war ended, hundreds of large bombers were suddenly surplus. Handley Page saw an opportunity. The firm’s new airline, Handley Page Transport, converted O/400s into early passenger aircraft — fitting cabins, windows and seats into the roomy fuselage — and from 1919 flew paying passengers between London and Paris, among the world’s first scheduled airline services. Civil versions carried designations such as O/7, O/10 and O/11. Early airline flying was dangerous work: the loss of a converted O/400 at Golders Green in December 1920 was one of the first fatal British airliner accidents, and it fed directly into the earliest air-safety regulation.

The design line also grew upward. The four-engined Handley Page V/1500 — a still larger bomber intended to reach Berlin from bases in England — was developed from the same thinking and was ready as the war closed. Together the O/400 and V/1500 mark the moment the heavy aeroplane became both a strategic weapon and a vehicle for civil aviation.

The O/400 is best understood alongside its German counterpart, the Gotha. In 1917 and 1918 German Gotha bombers, and the earlier Zeppelin airships, carried out daylight and night raids on London and other English towns. The material damage was limited, but the psychological and political effect was large: for the first time, an enemy could reach the British public directly, and the country demanded both a defence and a reply.

The O/400 was that reply in kind. Where the Gotha struck British cities, the O/400 — especially with the Independent Air Force — struck German industry and railways. The two aircraft were broadly comparable in concept: large twin-engined biplane bombers, operating mostly by night, carrying a similar order of bomb load. They represent the same idea arrived at on both sides — that the aeroplane could carry a war beyond the battlefield — and together they mark the true beginning of strategic bombing. The German raids also helped push Britain to create the RAF, the service that then flew the O/400 over Germany.

Flying an O/400 on a night raid was a feat of endurance. The crew of four or five sat in open or lightly enclosed positions, exposed to a freezing slipstream for sorties that could last many hours. There was no radar, no radio navigation and no reliable blind-flying instruments; the pilot flew by feel, by compass, and by the dim shapes of rivers, coastlines, forests and towns below.

Finding the target

Navigation was by dead reckoning — heading, airspeed and time, corrected for a wind that could only be guessed at — checked against whatever landmarks moonlight revealed. Reaching the right town at all was an achievement; hitting a specific factory or rail yard within it, with the simple bomb sights of the day, was harder still. Anti-aircraft fire and the ever-present risk of engine failure over enemy territory made every trip a gamble.

These conditions explain both the O/400’s strengths and its limits. It was steady, roomy and long-legged — good qualities for a night bomber — but the accuracy and weight of effort it could deliver were bounded by the technology of 1918, and honest histories of the Independent Force stress how modest the physical results were compared with the scale of the idea.

One of the more remarkable episodes in the type’s career took place far from the Western Front. In 1918 a single long-range O/400 was flown out to Egypt by way of a long ferry route — a demanding journey for any aircraft of the period — to reinforce the air effort in the Middle East. Its size alone made it a sensation among crews used to far smaller machines.

No. 1 Squadron, Australian Flying Corps, operated the type from Haifa in the last phase of the Palestine campaign, using its bomb-carrying capacity against Turkish and German airfields, troop columns and communications as General Allenby’s forces advanced north. The presence of a heavy bomber in that theatre, however briefly, showed the reach the design gave the RAF and its Dominion squadrons, and it is one reason the O/400’s story spans not just France and Germany but the eastern Mediterranean as well.

The O/400’s front-line career was short. Within a year of the Armistice the RAF was replacing it with the Vickers Vimy, a cleaner and more capable heavy bomber, and the wartime O/400 fleet was rapidly run down. Some airframes found a second life in civil hands as airliners and survey aircraft; most were simply scrapped as surplus.

That is the core reason no complete example survives. In 1919 the O/400 was yesterday’s weapon, produced in haste for a war that had just ended, and there was neither the appetite nor the money to preserve one. By the time aviation history was valued enough to warrant museum collections, the intact aircraft were long gone. What remains — relics, sections and fittings in collections including the Royal Air Force Museum — is precious precisely because it is all that is left of an aircraft that mattered a great deal and vanished almost completely.