Background and original specification
In 1953, as
British European Airways (BEA) introduced the world's first
turboprop-powered civil airliner – the
Vickers Viscount – into passenger service, the operator was already considering what would be required of a potential successor.
 Following the entry into service of jet airliners in 1952, many airline managers and economists remained sceptical and advocated turboprop airliners as replacements of piston-engined airliners.
 In 1953, while several manufacturers across the world were investing in pure jet-powered aircraft, BEA chose to favour turboprops on the basis of their superior economics and produced a specification that called for an aircraft capable of seating 100 passengers and attaining a maximum speed of 370 knots.
 As a result of BAE's specification, Vickers developed an enlarged derivative of the Viscount for BEA, the
Vickers Vanguard, which was ordered by the airline on 20 July 1956. By this point, however, the French-built
Sud Aviation Caravelle conducted its
maiden flight during the previous year and BEA was beginning to recognise that jet aircraft could soon be providing stiff competition.
In April 1956, Anthony Milward, chief executive of BEA stated that he "would rather do without [jet airliners]". Nevertheless, in December of that same year,
Lord Douglas of Kirtleside, BEA's chairman, stated that it might be necessary for a number of jet-powered short haul aircraft to be introduced while retaining turboprop aircraft as the mainstay of the company's inventory for the foreseeable future.
 In July 1956, BEA had announced what it called "outline requirements" for a short-haul "second generation jet airliner", to work alongside its turboprop fleet. It would carry a payload of some 20,000 lb (9,100 kg) or some 70 passengers over up to 1,000 miles (1,600 kilometres), weigh about 100,000 lb (45,000 kg), use 6,000 ft (1,800 m) runways, cruise at a very high speed of 610 to 620 mph (980 to 1,000 km/h) and have "more than two engines".
 According to aviation author Derek Woods, BEA "wanted something that was faster than the Caravelle which was threatening to be highly competitive".
 While not intended as an express requirement, commentators ever since have taken these figures to constitute a definite call to industry.
Four companies prepared projects to match the BEA outline.
Bristol proposed the initially-four-engined
Bristol Type 200.
Avro proposed the futuristic
Avro 740 trijet before shelving it and joining forces with Bristol and Hawker Siddeley.
Vickers proposed the VC11 four-engined airliner, derived from its in-development
VC10. The de Havilland company considered three possible contenders for the specification; two of these were four-engined developments of the early
Comet, the world's first jet-powered airliner: the D.H.119 and the D.H.120, the latter being also intended to be offered to
British Overseas Airways Corporation (BOAC). In July 1957, de Havilland made another submission in the form of the D.H.121; this proposal was furnished with three
turbojet engines and greatly resembled the eventual production aircraft.
 By August 1957, the D.H.121 proposal had been revised; differences included the adoption of the in-development
turbofan engine, and its expansion to accommodate a maximum of 98 passengers.
The D.H.121 was to be the world's first
trijet airliner. Its designers felt this configuration offered a trade-off between cruising economy and take-off safety in case of an engine failure; moreover, the BEA specification had called for "more than two engines". Each of the three engines would drive its own hydraulic system, offering triple redundancy in case of any of the other systems failing. The engines were to be 13,790 lbf (61.34 kN) Medway engines. The D.H.121 was to have a
gross weight of 123,000 lb (56,000 kg) or optionally, up to 150,000 pounds (68,000 kg), a range of 2,070 miles (3,330 kilometres), and seating for 111 in a two-class layout (or for over 140 in a high-density single-class layout as typical from the 1960s onwards on inclusive-tour
 The design initially included a
cruciform tail layout similar to that of the Caravelle.
 The engines were clustered at the rear, with the centre engine situated in the extreme rear of the fuselage fed by air ducted through a large oval intake at the front of the fin, a configuration similar to the later Boeing 727; the design eventually settled on a variable-incidence T-tail.
From the outset, the D.H.121 was planned to employ
avionics which were very advanced for the period. Among other capabilities, they would offer automatic approach and landing within a few years of service entry. The avionics were also to have triplicated components for reliability and to allow "majority 2:1 voting" for aircraft guidance during automatic approach and landing.
 The physical dimensions of most avionics of the period required them to be housed in a large compartment beneath the Trident's flightdeck; the compartment's size was among the factors dictating a distinctive nose
undercarriage design: offset by 2 ft (61 cm) to the port side and retracting sideways to stow across the D.H.121's longitudinal axis.
Industry consolidation and selection
BEA soon selected the D.H.121 as the basis for detailed negotiations, but these talks were protracted due to multiple factors, including wider policy decisions of the British government and indecision within BEA itself.
 During the time that the D.H.121 had emerged in the late 1950s, the British government came to view the airframe and
aeroengine industries as too fragmented into small companies; accordingly, a policy favouring mergers into a few large groups was adopted. de Havilland was keen to retain their independence and leadership of the D.H.121, and thus approached the government with a proposal to form a consortium under which de Havilland would produce the fuselage, Bristol would manufacture the wings, and various other companies, including
Hunting Aircraft and
Fairey Aviation would be responsible for other elements; however, Bristol strongly opposed this arrangement and chose to work with
Hawker Siddeley in competition against de Havilland.
Companies vigorously competed to be selected by BEA due to the lure of its £30 million contract, as well as the likelihood of lucrative overseas export sales.
 On 4 February 1958, de Havilland, along with Hunting and Fairey, announced that they had agreed to form a partnership for the purpose of manufacturing and marketing the D.H.121; the consortium adopted the corporate name of the defunct
Airco company, which had been
Geoffrey de Havilland's employer during the
First World War.
Minister of Supply stated of the Airco consortium that "this is not quite what [he] had in mind". Nevertheless, both Airco and the rival Bristol-Hawker Siddeley team proceeded to conduct their own approaches to various overseas airlines; sufficiently interested, American airline
Pan American World Airways invited both teams to present their proposed airliners in January 1958.
Matthew Slattery, chairman of Bristol and
Short Brothers, appealed for BEA to delay any decision until after one of the competing firm had already secured an export order for their airliner. In response, Lord Douglas stated that BEA wished to order the D.H.121 and was awaiting approval from the government; Douglas's reply has been viewed as the
death knell for the rival Type 200 proposal.
Meanwhile, a new prospective rival airliner emerged, this time from
Boeing in the United States, in the form of the
Boeing 727, which also had a tri-jet configuration.
 Boeing had began its studies into this sector of the market in 1956, and elected to launch its own tri-jet programme in 1959. Airco executives, who were at the time intensely exploring various alternatives and further partnerships with other aircraft companies, considered the possibility that Boeing may choose to drop the 727 project and instead co-manufacture the Airco D.H.121 in the USA; Lord Dougal was one of the proponents of this initiative.
 As a result, Airco invited a team of Boeing engineers and executives to
Hatfield, Boeing later permitted a return visit by de Havilland representatives to Seattle; however, Boeing revealed little detail or their plans for the 727, while virtually all information on the D.H.121 had been shared with Boeing, an openness that had allegedly "amazed" Boeing.
 British commentators have tended to interpret this episode as involving the acquisition of sensitive proprietary data on the D.H.121 by a direct competitor.
 Woods described the action as "de Havilland solemnly handed over all its research over to its rivals...the crowning piece of stupidity".
On 12 February 1958, the British government authorised BEA to commence contractual negotiations along with the issuing of a
letter of intent for 24 aircraft.
 Accordingly, that same month, BEA announced that the D.H.121 had come closest to its requirements and that it would proceed to order 24 with options on 12 more.
 It took a further six months for the government to approve a formal BEA order for the D.H.121; the government had favoured the Bristol 200 for industrial policy reasons.
 Reportedly, BEA had a considerable interest in the Caravelle itself, however this would have been a politically unacceptable choice.
 BEA also favoured de Havilland, and therefore the Trident submission, due to the firm's established experience with jet airlines with its prior development of the Comet.
In April 1958, de Havilland firmed the general configuration of the D.H.121 and established a development timetable, including a projected date for the type's
maiden flight to be conducted during mid-1961.
 The company's market research department was forecasting that as many as 550 airliners in its category would be sold by 1965. Noting that a greater preference for the seating dimensions of what would become
Economy class was emerging amongst airlines, design alterations were made to adopt a slightly larger diameter fuselage to accommodate six-abreast seating, providing for a maximum configuration of 111 seats.
 According to Woods, this enlarged version of the D.H.121 was "on the verge of building the right aeroplane for the market and the success of the Viscount looked like being repeated".
Hawker Siddeley Trident 3B
In March 1959, BEA, which had become concerned by a recent decline in passenger growth, concluded that the D.H.121's payload-range capacity could be too great for their needs and petitioned de Havilland to reduce the scale of the design to suit their revised projections.
 Fearing that the proposed scale of the Trident was too large, the airline had elected to effectively tear up the programme for its redesigning for their immediate situation. In 1959, BEA had a large fleet in operation and on order, and the issue of overcapacity was a critical concern.
 The airline's concerns reflected three factors: a short-lived airline recession in the late 1950s; the imminent arrival into service of a large fleet of turboprop
Vickers Vanguards which duplicated the D.H.121's general payload-range area, and the growing trend to higher-density seating.
Although de Havilland stated that they generally concurred with BEA, its management also stated that they had worked "under terms more onerous than anything D.H. had previously undertaken".
 Industry observers at the time felt that the British aircraft industry had again stumbled "into the pitfall of having designed exclusively for one customer an aeroplane that has potentially a much wider scope":
 a sentiment which would be echoed throughout the Trident's subsequent history. The de Havilland board elected to submit to BEA's demand, overriding input from its own sales and market research departments which indicated that other airlines sought the larger model instead.
 It was, however, noted that de Havilland had not yet secured a formal and final BEA order and that its competitor Bristol was actively promoting their 200
[N 3] project, which was significantly smaller than the D.H.121. At the time
Douglas were also downsizing their
727 projects. It was felt the original large D.H.121 would have to compete against the
Convair 880 and
Boeing 720 some four years after their service entries, whereas a cut-back design would be more competitive against the then-projected 75–100 seat, twin-engined DC-9.
Downsizing the Trident involved substantial changes to the design being made, including a powerplant change from the Medway to a scaled-down derivative, the 40 percent less powerful 9,850 lbf (43.8 kN)
Rolls-Royce Spey 505.
 The gross weight was cut by about a third to 105,000 pounds (48,000 kilograms), while the range was cut by more than half to 930 miles (1,500 kilometres), and mixed-class seating was cut by about a quarter to 75 or 80 (97 in a single-class layout). Wing span was reduced by approximately 17 ft (5.2 m), wing area by 30 percent and overall length by 13 ft (4.0 m). The revised design retained some features of the original one, notably its
fuselage diameter. It had a smaller flightdeck and single-axis, two-wheel, four-tyre main undercarriage legs in place of four-wheel bogies.
 Woods summarised the BEA-mandated redesign as: "At one blow the 121 was emasculated in terms of size, power and range".
Six months following BEA's request, de Havilland and the airline came to an agreement on the downsized D.H.121.
 Details of the emerging aircraft, including its pioneering avionics, were announced to the public in early 1960.
 It was this revised aircraft that BEA ultimately ordered on 24 August 1959, initially in 24 examples with 12 options.
 In September 1960, the future airliner's name, Trident, was announced at the
Farnborough Airshow; this name had been chosen as a reflection of its then-unique three-jet, triple-
Further development and proposals
By 1960, de Havilland had been acquired by the Hawker Siddeley group.
 After the de Havilland takeover, Airco was disbanded. Hunting was marshalled into the competing newly formed
British Aircraft Corporation (BAC); their departure removed any putative possibility of the Hunting 107 (later the
BAC One-Eleven) being marketed alongside the D.H.121 as a complementary, smaller member of the same airliner family. Fairey Aviation, partially incorporated into
Westland Aircraft, also left the D.H.121 project.
[N 4] With the move to Hawker Siddeley Aviation, the designation was eventually revised to the HS 121. The reorganisation of the industry had compounded upon the delays caused by BEA's changes to the specification, which had in turn harmed the Trident's competitiveness against the Boeing 727.
The rival Boeing 727 had quickly established a lead over the Trident.
 The 727's early lead only strengthened it in subsequent competitions; one such example is
Trans Australia Airlines, which had determined the Trident to be superior to the Boeing 727 from an operational standpoint, however it was also viewed as having been commercially risky to choose a different fleet from rival airlines such as
Ansett Australia, who had already selected the 727.
 By 1975, only 117 Tridents had been sold against over 1,000 727s.
According to Woods, a significant opportunity that may have enabled the Trident to catch up with the 727 was lost during the 1960s in the form of two competitions for a
maritime patrol aircraft; a
NATO design competition to replace the
Lockheed P-2 Neptune, and
Air Staff Requirement 381, which sought a replacement for the
Royal Air Force's piston-engined
 Amongst the various submissions that had been produced in response was a bid by
Avro, which had become another member of the Hawker Siddeley group by that time, which was designated as the Avro 776. The proposed Avro 776 mated the Trident's fuselage with a redesigned and enlarged wing along with more powerful
Rolls-Royce RB178 engines capable of 16,300 lb of thrust.
 In addition to the maritime patrol requirement, Avro envisioned that the aircraft could be utilised in various military roles, including as a 103-seat troop transport as well as being armed with up to four
air-launched ballistic missile (ALBM) as a
 In addition to Avro's proposals,
Armstrong Whitworth had also proposed their own military variants of the Trident.
Later revisions of the Avro 776 substituted the RB.178 engine for the newer
turbofan engine, the development of the latter being supported by the 776's procurement if selected.
Rolls-Royce Limited, having shelved development of the Medway following the Trident's redesign, was keen to develop an engine to slot between the 10,000 lb Spey engine and the 20,000 lb
Rolls-Royce Conway engine; if such an engine had been produced, it could have equipped new versions of the civil Trident as well. Furnished with a more capable engine that could provide more thrust than the Spey was capable of, an extended fuselage could also have been adopted and existing landing restrictions could have been discarded; overall, the Trident would have been a far closer match to the 727.
 Wood summarised the importance of this prospective development as: "For the Trident programme, the RB.177 would have been a God-send".
At one point, the Avro 776 looked set to win the competition to be selected as the RAF's new maritime patrol aircraft.
 However, due to a desire to cut costs, the RAF decided to issue an entirely new Operational Requirement, under which the demands for speed, endurance, and capacity had all been diminished. As a result of the changed requirement, the design team was recalled and the Avro 776 was entirely sidelined for a new proposal.
 This new proposal, based upon the de Havilland Comet's fuselage, had little to do with the Trident save for the use of its existing Spey engines; this would go on to be selected and procured as the
Hawker Siddeley Nimrod. As a result of this loss, prospects for an enlarged Trident equipped with more powerful engines effectively evaporated.