The Short S.25 Sandringham is a British civilian flying boat designed and originally produced by Short Brothers. They were produced as conversions of the widely used Short Sunderland, a military flying boat that was commonly used as a maritime patrol aircraft.

Prior to the Sandringham, numerous Sunderlands which had been built for the Royal Air Force (RAF) had been converted to a civilian configuration as early as 1942 to fulfil a requirement of the British Overseas Airways Corporation (BOAC) for additional long range airliners. This need led to the development of the Short Hythe, which was a somewhat austere conversion due to its development taking place amid the Second World War. Around the conclusion to the conflict, Shorts was keen to produce a more refined and capable conversion of the Sunderland; the first prototype of which, performed its maiden flight during November 1945, became known as the Sandringham I.

The BOAC quickly sought to procure the Sandringham for its fleet, the demand for which led to production of the Sandringham II along with multiple further improved versions. Being operated initially alongside the Hythes, the more comfortable conditions onboard the Sandringhams led to them being given greater priority for usage and routing alike. Various other operators also procured the type, including TEAL, Qantas and Ansett. The Sandringham soon became outclassed by land-based airliners, including the arrival of faster jetliners, thus their operations curtailed substantially during the late 1950s. The final scheduled service performed by the type was ended during the 1970s. A total of three Sandringhams are believed to have survived to the present day in preservation.

At the height of the Second World War, it was recognised that the UK's flagship airline, the British Overseas Airways Corporation (BOAC), required new aircraft in its inventory. It was promptly recognised that, as the Short Sunderland military flying boats operated by the Royal Air Force (RAF) bore considerable similarities to the Short Empire, a civilian flying boat, there were few challenges posed by converting excess Sunderlands for civil purposes if deemed necessary. Accordingly, during December 1942, work started on six Sunderland IIIs from Short's Rochester works, stripping them of their armaments and military fittings, being instead refitted for airline purposes, having austere bench-type seats installed within the cabin along with civil markings and registrations across their exteriors. On 26 December 1942, the first of the conversions performed its maiden flight. After being delivered to BOAC, these flying boats were quickly used on the airline's Poole-West Africa service, transporting both passengers and air mail alike.

The initial batch having been determined to be satisfactory, a further six Sunderlands were converted for similar purposes in 1943, along with a further batch of 12 during the following year. Following the end of the Second World War, BOAC opted to convert its Sunderlands to a less-austere standard, making them more suitable for peace-time operations; they became known as the Short Hythe. Specific changes including the replacement of the somewhat primitive bench seats with individual seats, initially permitting up to sixteen passengers to be accommodated on one deck in the initial H.1 configuration. Improved models, such as the H.2 variant, featured the addition of a promenade deck, while the H.3 configuration, featured an additional eight seats. Up to 6,500 pounds (2,900 kg) of mail could also be carried. Engines were standardised as the Bristol Pegasus 38.

Even prior to the end of the conflict, it had been identified that, while the converted Sunderlands had proved to be successful, there were areas for improvement. Specifically, the temporary and somewhat basic fairings implemented to cover turret positions could be replaced by more sophisticated low-drag counterparts to improve aerodynamic efficiency. Deciding to proceed with this work, in November 1945, Shorts thus flew a refined conversion of BOAC's Sunderlands from their Rochester works. The conversions commenced with removal of their armaments and military fittings, removal of the bow and stern sections and then removal of the paint down to the bare metal before the extent of the corrosion was assessed and treated before reskinning of a considerable amount of the below waterline structure to restore its watertightness. Along with the revised low-drag fairings that were installed on both the nose and tail, a refurbished interior was installed. This flying boat was referred to as the Sandringham; it subsequently became known as the Sandringham 1 to distinguish it from the more advanced conversions which had interiors customized to each customer, which followed it.

During January 1946, a certificate of airworthiness was issued for the Sandringham I, it entered service with BOAC in June of that year after completing operational trials with RAF Transport Command. Around this time, BOAC opted to have all of its Sunderlands refurbished to a standard akin to contemporary airliners. Shorts had envisioned BOAC issuing a prompt order for Sandringhams, but the first order for the type came from the Argentine airline Compañía Argentina de Aeronavegación Dodero instead. The airline had been keen to procure Sunderlands for its South American intercity routes, and had been impressed by information provided by Shorts on the Sandringham conversion. Ordered in two batches, one for short-haul routes and the other for longer distances, the first Sandringham II was launched to great ceremony at Belfast on 17 November 1945.

There were several differences between the Sandringham I and the following production flying boats. While the first prototype had retained the Pegasus engines, common to both the Sunderland III and Hythe, later models of the Sandringham, which were converted by Short and Harland Ltd at Belfast Harbour, were based on the later Sunderland V, which were instead powered by Pratt & Whitney "Twin Wasp" engines. Every Sandringham was converted from surplus Sunderlands that had been formerly operated by RAF Coastal Command.