The engines were to be 13,790 lbf (61.34 kN) Medway engines. The D.H.121 was to have a gross weight of or optionally, up to , a range of , 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 charter flights). 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, with the nose landing gear offset by to the port side and retracting sideways to stow across the D.H.121's longitudinal axis. 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.