Shortly after the type had been retired by the RAF, five Belfasts were sold and placed into civilian service with the cargo airline TAC HeavyLift.

[citation needed] The Belfast has its origins in studies conducted by aircraft manufacturer Short Brothers into the possibility for matching an arrangement of four Bristol Orion turboprop engines with an airframe that had been optimised for the purpose of transporting various military stores during the mid-1950s.

[4] In March 1957, Shorts submitted its first brochure on their proposed transport aircraft, designated as the PD.18 and given the name Britannic to reflect its Britannia ancestry.

[4] As the design of the proposed aircraft was repeatedly revised, it had less and less in common with the earlier Britannia, incorporating a greater proportion of all-new components, sections, and systems.

The first two aircraft produced were equipped with dedicated flight-test instrumentation and were used to complete development trials, which totalled roughly 850 flight hours; certification was conducted to both RAF and Air Registration Board (ARB) requirements.

[5] The SC5/10A was to have been a commercial freighter derived directly from the Belfast C.1, retaining much of its design such as its large rear loading doors for access to the main freight hold.

[5][12] The second civil variant to be offered, the SC5/31, was to have featured a swinging nose arrangement for loading purposes in place of the rear door layout used upon the RAF model of the aircraft.

[5][3] During 1964, Short revealed that it had approached British European Airways (BEA) with its proposal for a large double-deck short-haul passenger version of the Belfast.

The SC5/35 model was intended to be a strategic freighter capable of conducting very long range missions, while the SC5/15 was an envisioned tactical transport configuration, the latter of which was claimed to have required very little modification from the existing aircraft.

[12] For this variant, the fuselage of the Belfast would have been paired with the wing of the Lockheed C-141 Starlifter, which would have readily enabled the adoption of turbojet engines in place of the turboprops.

Speculated engines to power this variant included the Pratt & Whitney JT3D-3 (18,000 1b thrust) or JT3D-8 (21,000 1b), Rolls-Royce Conway 550 (21,825 1b) or Bristol Siddeley BS.100 (27,000 1b approximately).

[15] Detailed presentations on the SC.5/41 and SC.5/45 proposals were made to British Overseas Airways Corporation (BOAC) and RAF respectively, but no military order was forthcoming.

According to aerospace publication Flight International, the design of the assemblies of the surfaces of both the tail and wing of the Belfast had been derived from the Bristol Britannia.

[18] Rolled Z-sections were used throughout the majority of the fuselage frames and stringers, while box beams are used where the exertion of heavier-than-average loads had been typically anticipated; the structure lacks any use of forgings or machined members.

[18] The Belfast was equipped with a full automatic landing system, produced by Smiths Aerospace, the first aircraft in the world to be designed to feature such capability from the onset of development.

[3] The autopilot and flight control system, known as the ASR 518, was triplex in the roll and pitch channels, with duplex or emergency simplex ability.

The Belfast was capable of a maximum takeoff weight (MTOW) of over 220,500 lb (100 tonnes) – less than the contemporaneous 250-tonne Antonov An-22 and the 128-tonne Douglas C-133 Cargomaster, but more than the Lockheed C-130 Hercules.

Modifications and testing were subsequently carried out, particularly on aircraft SH1818 (which was at the time perfecting the RAF's requirement for CAT 3 automated landings at RAE Bedford), and a new rear fairing was developed, which had the result of raising the fleet's cruising speed by 40 mph.

[20] HeavyLift's Belfasts were again contracted to support the RAF during the first Gulf War, transporting vehicles and helicopters too large to be carried by the Hercules fleet.