In particular, Imperial Airways' technical adviser, Major Robert Hobart Mayo developed a specification for a new type of aircraft to serve both passenger and freight requirements throughout the world.

[2] According to aviation author Geoffrey Norris, at the time, it was considered to be impossible to construct a land-based aeroplane of such size and weight that still possessed acceptable landing/take-off performance, thus it was determined that a flying boat would be required.

[2] Early on, it was apparent that Short Brothers, which had previously developed and produced several large flying boats for Imperial Airways and the Royal Air Force (RAF) that had proven to be sound designs in terms of performance and safety, would be a frontrunner to fulfil the requirement.

[2] On 4 July 1936, the first Empire flying boat to be built, G-ADHL, named Canopus, conducted its maiden flight, flown by Shorts' chief test pilot, John Lankester Parker.

[5] Early flights of the type were relatively trouble-free, Parker declaring his satisfaction with the aircraft's performance; the new styling of the planing bottom used also quickly proved its value.

[6] The first series of the Short Empires, the S.23, could carry five crew, 17 passengers, and 4,480 lb (2,035 kg) of cargo at a maximum speed of 174 knots (320 km/h) and was powered by four 920 horsepower (690 kW) Bristol Pegasus Xc radial engines.

[7] The second Empire and the first of the long-range models to be produced, G-ADHM, named Caledonia, performed her first flight on 15 September 1936 and was delivered to Imperial Airways on 4 December 1936.

[10] It also had a strengthened airframe and used heavier gauge sheeting on the fuselage and wings; while these changes allowed the takeoff weight to be increased to 46,000 pounds (21,000 kg) with a corresponding range of 1,500 miles (2,400 km), the S.30 had broadly the same performance as the preceding S.23 flying boats.

[11] The aircraft had similar appearance to the standard Empire, but was in fact roughly about 15 percent larger in all dimensions, as well as differing in its use of the more powerful Bristol Hercules radial engine.

[12] The Short Empire flying boat was a relatively clean-looking high-wing monoplane, initially powered by an arrangement of four wing-mounted Bristol Pegasus Xc radial engines which drove de Havilland-built variable-pitch propellers.

Elements of the leading edge of the wing, set upon either side of the engine nacelles, could be folded down to act as servicing platforms for both the engines and the floats, the latter of which were carried upon tandem struts fixed onto the main spar and featured shock absorbers in order to eliminate undue torsional loads being transmitted to the wings by the sudden impact of waves while travelling at speed.

The structure used a combination of Z-shaped stringers and I-shaped girders to form triangular sections that ran along the chines located at the point where the fuselage sides met with the planing underside.

[17] The lower deck contained a large marine compartment containing an anchor, a pair of drogues, a mooring bollard, and a boat hook, along with a step ladder to the cockpit; aft of the mooring compartment was the forward passenger saloon, followed by a central corridor flanked by the toilets and galley, a mid-ship cabin, a spacious promenade cabin, and finally an aft-cabin.

In 1941, two Empire flying boats – Clio and Cordelia returned to Shorts' Belfast facility, where they were modified with the addition of gun turrets in dorsal and rear positions and air to surface vessel (ASV) radar equipment installed on the top and sides of the fuselage.

[20] This flight, which covered a distance of roughly 2,300 miles (3,700 km) non-stop, showed that Britain could move military material to its overseas bases by air.

[20] On an experimental basis, Caledonia, piloted by Captain W N Cummings, flew a 1,993 miles (3,207 km) route from Foynes on the River Shannon, Ireland west to Botwood on the Bay of Exploits, Newfoundland.

[20] Caledonia took just over 15 hours (including a period spend searching for a landing spot), flying at an altitude of 1,500 to 5,000 ft (460 to 1,520 m) to cover 1,993 miles (3,207 km)—an average speed of about 130 mph (210 km/h).

Two boats (Caledonia and Cambria) were lightened and furnished with long-range tanks; both aircraft were used in experimental in-flight refuelling trials in order for them to conduct the journey; these modifications came at the cost of being able to carry fewer passengers and less cargo.

This concept had been strongly advocated for by Imperial Airways' technical adviser, Major Robert Hobart Mayo, as a means of significantly increasing both range and payload, and had been well received by both the airline and the British Air Ministry, the latter of which placed an order with Shorts.

[25][24] On 21 July 1938, a successful mid-air launch of Mercury was executed off the west coast of Ireland while carrying a 600 lb payload of mixed cargo and mail; it arrived at Montreal, Canada, 2,860 miles (4,600 km) 22 hrs 22 min later, having achieved an average speed of 141 mph (227 km/h).

For pilots trained on smaller less sophisticated aircraft judgement of height was difficult due to the high cockpit of the Empire, as well as the concept of using flaps to manage speed.

Once in service the structure was found to be weak in places, especially on the planing bottoms, which led to later models employing thicker gauge skins on the hull and wings.