Starting its history in 1956, the BAC One-Eleven originates from a 32-seat turbojet-powered transport which was designed as a project by Hunting Aircraft. Then identified as the Hunting H.107, it was planned with a power-plant of two rear-mounted Bristol Orpheus 12B turbojets, but after wind tunnel evaluation the design was amended to incorporate turbofans, then in the development stage. This led to a delay of four years, by which time Hunting Aircraft had been acquired by the British Aircraft Corporation, and it was decided to resurrect the H.107 for further market research and development by the combined design teams of Hunting and Vickers at Weybridge. There was little commercial enthusiasm for the BAC.107 in its final Hunting configuration, with accommodation for 59 passengers, but there was sufficient interest for a version with a maximum of approximately 80 seats to warrant the construction of a prototype and static test airframes.

Designated BAC.111 (and later to be called One-Eleven), the finally revised aircraft incorporated a circular-section all-metal pressurised fuselage, low-set swept monoplane wings incorporating Fowler type trailing-edge flaps, and airbrakes/spoilers on the wing upper surface, forward of the flaps. The T-tail included a variable-incidence tailplane, and the landing gear, of hydraulically retractable tricycle type, had twin wheels on each unit. Accommodation was provided for a maximum of 79 passengers in five-abreast high-density seating, and in addition to a conventionally placed passenger door at the forward end of the cabin on the port side, the BAC.111 had also a ventral airstair below the tail unit, giving access to or from the aft end of the cabin. Powerplant of the prototype One-Eleven Series 200, which was intended as the basic production version, consisted of two 4722kg thrust Rolls-Royce Spey Mk 506 turbofans, and so powered the prototype flew for the first time on 20 August 1963. Two months later, on 22 October, this aircraft was lost during the flight development programme, together with a highly experienced crew of seven that included test pilot M. J. Lithgow. Investigation showed the cause to be a deep stall, resulting from the T-tail and rear-mounted engine configuration, and remedial action included the installation of powered elevators, a stick-pusher, and modification of the wing leading edges. These changes were adequate to prevent the aircraft from assuming an inadvertent and dangerous angle of attack, a condition peculiarto this configuration, in which the wing loses lift and the horizontal tail surfaces are unable to restore longitudinal stability. Although responsible for extending considerably the test and development programme of the One-Eleven (the full certificate of airworthiness not being awarded until 5 April 1965), the detailed investigation of the cause and remedy of the deep-stall phenomenon was to prove of considerable value to aircraft designers and manufacturers worldwide.

Long before certification, in May 1963, the British Aircraft Corporation (BAC) announced that it was intended to develop two other versions in addition to the basic Series 200. These were to include an increased payload/range One-Eleven Series 300, with 5171kg thrust Spey Mk 511 turbofans, and a generally similar One-Eleven Series 400 that would incorporate modifications to meet US requirements. As well as introducing more powerful engines, the Series 300 had increased fuel capacity, and strengthened wings and landing gear to cater for a 3856kg increase in gross weight. Interest in the One-Eleven was growing, following the initial order of 10 Series 200 aircraft from British United Airways (BUA), and market potential within the USA was demonstrated by an early order for six aircraft from Braniff International. When this was followed by orders from other US carriers, including American Airlines, the prospect for fairly large US sales seemed very good. However, by the time that FAA Type Approval was awarded, on 16 April 1965, there was a growing number of aircraft competing within the same payload/range category, and total sales to US carriers failed to reach the figures that had at one time seemed possible. Initial One-Eleven services were flown by British United, from Gatwick to Genoa, on 9 April 1965; in the USA, Braniff's first Corpus Christi-Minneapolis revenue flight was made on 25 April. In January 1966, BUA inaugurated London-Scotland and London-Northern Ireland One-Eleven domestic routes. Production of the three initial versions of the One-Eleven totalled 134: 56 Series 200, nine Series 300, and 69 Series 400 aircraft.

The steadily increasing number of air travellers meant that, within most categories of aircraft, carriers were looking for greater accommodation/payload capacity. 'Stretched', or increased-capacity, versions of the One-Eleven had been under consideration by BAC at much the same time as the original Series 200/300/400 aircraft were announced. However, it was not until British European Airways (BEA) began to show interest in an enlarged One-Eleven that design of what was to become the One-Eleven Series 500 was finalised. With a fuselage lengthened by 2.54m forward of the wing, and 1.57m aft of the wing, the Series 500 could accommodate a maximum of 119 passengers. More powerful engines were introduced, the wing span increased by 1.52m, and the structure of both landing gear and wings strengthened to make possible a significant increase in gross weight. This was originally 41277kg for takeoff, but has since been raised to a maximum of 47400kg. The prototype for the Series 500 was produced by conversion of the Series 400 development aircraft, and this flew for the first time in its new configuration on 30 June 1967. ARB certification of a production example was gained on 15 August 1968, and BEA's first revenue flight was flown three months later, on 17 November.

The final variant to appear was the One-Eleven Series 475, intended for operation from and into smaller airports, or in higher temperature/altitude environments. This retains the standard fuselage/accommodation of the Series 400, and combines the powerplant and wings of the Series 500, plus a modified landing gear to permit operation from lower-grade surfaces.

In addition to Series 475 and 500 aircraft which were available from BAe in standard configuration, two other special variants were available. These comprised executive or freighter configurations, and approximately 40 examples of the former are in service worldwide. The freighter conversion includes installation of a 3.05 by 1.85m hydraulically actuated cargo door in the port forward fuselage, and a quick-conversion freight handling system. New-technology options for new aircraft, and in some cases suitable for retrospective installation, include a fully certificated Category II automatic landing system, automatic throttle control, and engine 'hush-kits'.

A total of 230 One-Elevens had been built by BAC/BAe by the time production ended in Britain. However, responsibility for the type had moved to Romania, where the aircraft is still in production. Two variants are built, these being the Series 495 and Series 560, Romanian-built variants equivalent to the Series 475 and 500 respectively. Total transfer of production was preceded by the delivery in 1981-82 of a Series 487 freighter and two Series 525s. Then 22 kits were supplied to be assembled by the newly-established ROMBAC company. The first Romanian-assembled aircraft, a Series 560, flew for the first time on 18 September 1982 before delivery to the state airline TAROM. About twenty have been completed so far.