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.

By this point, however, the French-built Sud Aviation Caravelle had conducted its maiden flight during the previous year, and BEA was beginning to recognise that jet aircraft could soon be providing stiff competition.

[4] 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.

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 DH.121; this proposal was furnished with three turbojets, Rolls-Royce Avon engines, and greatly resembled the eventual production aircraft.

[9] By August 1957, the DH.121 proposal had been revised; differences included the adoption of the in-development turbofan, the Rolls-Royce Medway, and an expansion to accommodate a maximum of 98 passengers.

[12] The physical dimensions of most avionics of the period required them to be housed in a large compartment beneath the Trident's flight deck; the compartment's size was among the factors dictating a distinctive nose undercarriage design, with the nose landing gear offset by 2 ft (61 cm) to the port side and retracting sideways to stow across the DH.121's longitudinal axis.

[9] During the time that the DH.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.

[13] Companies vigorously competed to be selected by BEA due to the lure of its £30 million contract (equivalent to £885,295,000 in 2023[14]), as well as the likelihood of lucrative overseas export sales.

[15] Sir Matthew Slattery, chairman of Bristol and Short Brothers, appealed for BEA to delay any decision until after one of the competing firms had already secured an export order for their airliner.

Airco executives, who were at the time intensely exploring various alternatives and further partnerships with other aircraft companies, considered the possibility that Boeing might choose to drop the 727 project and instead co-manufacture the DH.121 in the USA; Lord Douglas was one of the proponents of this initiative.

[21] BEA also favoured de Havilland, and therefore the Trident submission, due to the firm's established experience with jet airliners with its prior development of the Comet.

[9][21] In April 1958, de Havilland firmed the general configuration of the DH.121 and established a development timetable, including a projected date for the type's maiden flight to be conducted during mid-1961.

[16] In March 1959, BEA, which had become concerned by a recent decline in passenger growth, concluded that the DH.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.

The de Havilland board elected to submit to BEA's demand, over-riding input from its own sales and market research departments, which indicated that other airlines sought the larger model, instead.

[16] Notably, de Havilland had not yet secured a formal and final BEA order and its competitor Bristol was actively promoting their 200 [N 2] project, which was significantly smaller than the DH.121.

It was felt the original large DH.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.

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.

[36] In addition to the maritime patrol requirement, Avro envisioned that the aircraft could be used in various military roles, including as a 103-seat troop transport and as being armed with up to four GAM-87 Skybolt air-launched ballistic missiles as a nuclear-armed bomber.

Furnished with a more capable engine that could provide more thrust than the Spey could, 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.

[41] Due to a desire to cut costs, though, the RAF decided to issue an entirely new operational requirement, under which the demands for speed, endurance, and capacity had all been diminished.

[41] 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.

[41] Hawker Siddeley Aviation, which had absorbed de Havilland, needed additional customers for the Trident, so entered into discussions with American Airlines (AA) in 1960.

In response, design began on a new Trident 1A, powered with up-rated Rolls-Royce Spey 510 engines of 10,700 lbf (47.6 kN) thrust, and a larger wing with more fuel, raising gross weight to 120,000 lb (54,000 kg) and range to 1,800 mi (2,900 km), but AA eventually declined the aircraft in favour of the Boeing 727.

The main difference was a larger fuel tank in the centre section of the wing, raising weights to 115,000 lb (52,000 kg), and range to 1,400 mi (2,300 km).

Delays were commonplace when Category 1 (Cat 1 = 200 ft (61 m) decision height and 600 metre runway visual range RVR) instrument landing system (ILS) was in use.

This would be powered by 11,400 lbf (50.7 kN) Spey 511s, have a gross weight of 135,600 lb (61,500 kg), an increased wing area by extending the chord, leading-edge slats for improved field performance, different takeoff flap settings (a 23 degree setting shortened the runway length required but imposed a 59,900kg MTOW), and the same fuselage, but with up to 140 seats in a six-abreast configuration.

Now powered by newer Spey 512s with 11,930 lbf (53.1 kN) thrust, it also replaced wing leading-edge droop flaps with slats, and extended the span with Küchemann-style tips.

[54] The beginning of the Trident's end came in the early 1980s, since ICAO began drafting noise regulations that would require first- and second-generation jet airliners to fit hush kits to the engines.

British Airways, the type's main operator, saw the required refits as not viable and instead chose to phase the Trident out of their fleet in 1985, the final flights were made on 31st December 1985.