It started as a de Havilland project for BEA. Other companies became involved and it was to be built by Airco but the British Government intervened and de Havilland became part of Hawker Siddeley, other members of Airco became part of BAC so the grouping was broken up and the aircraft became an HS project.
By the time of the first flight in January 1962 the projected RR Medway engines had been dropped, BEA had changed its mind and downgraded its requirements to the detriment of the type and Boeing's engineers had been invited to Hatfield where they were shown the Medway powered plans, were impressed, and went away and built the B727.
The nose wheel was offset to the left and retracted sideways to allow room for avionics connected with the blind landing equipment for which the type was the world leader, making the first truly blind landing of any airliner in a 1965 trial and became the first to do so in commercial service in 1966.
The type:
At the time of development: DH121 Trident
At the time of first BEA order: Airco121 Trident
At the time of first flight and its final designation: HS121 Trident
The main undercarriage also had an oddity. On retraction the undercarriage leg first shortened, then the leg rotated the wheels through 90 degrees as the leg was retracted sideways into the fuselage/wing centre section.