Sharing several similarities with the S.23, it featured a more advanced aerodynamic hull and was outfitted with various offensive and defensive armaments, including machine gun turrets, bombs, aerial mines, and depth charges.

Numerous unarmed Sunderlands were also flown by civil operator British Overseas Airways Corporation (BOAC), traversing routes as far afield as the Pacific Ocean.

[4] During the post-war era, use of the Sunderland throughout Europe rapidly declined, while greater numbers remained in service in the Far East, where large developed runways were less prevalent.

In response, Imperial Airways announced a competition to design and manufacture a fleet of 28 large flying boats, each weighing 18 long tons (18 tonnes) and having a range of 700 mi (1,100 km) with a capacity for 24 passengers.

[8] During November 1933, the British Air Ministry released Specification R.2/33, which called for the development of a next-generation long-range general purpose flying boat, intended to perform ocean reconnaissance missions.

[9] The tail turret was also changed to a powered version; Gouge therefore had to devise a solution to account for the resulting movement aft of the aircraft's centre of gravity, which was initially achieved via the presence of ballast positioned in the forward area.

[10] On 16 October 1937, the initial S.25 prototype (K4774) performed the type's maiden flight, fitted with Bristol Pegasus X radial engines capable of generating 950 hp (710 kW) of power each.

[7][10] Following early flight trials with the first prototype, the aircraft was returned to the workshop where it underwent further modifications; the adoption of a wing sweepback of 4° 15' was achieved via the addition of a spacer into the front spar attachments.

[10] On 7 March 1938, K4774 conducted its first post-modification flight, having been outfitted with the intended Bristol Pegasus XXII radial engines, each one capable of generating 1,010 hp (750 kW).

[10] On 28 May 1938, this second aircraft, which had been cleared for operations under tropical conditions, flew a record-breaking flight to Seletar, Singapore, stopping off along the route at Gibraltar, Malta, Alexandria, Habbaniyah, Bahrain, Karachi, Gwalior, Calcutta, Rangoon, and Mergui.

Of these, the flaps employed unusual Gouge-patented devices that slid backwards along curved tracks, moving rearwards and downwards to increase the wing area and generating 30% greater lift for landing.

The specification to which the Sunderland was developed to conform with had called for an offensive armament of a 37 mm gun and up to 2,000 lb (910 kg) of bombs, mines or (eventually) depth charges.

Defensive armament included a Nash & Thompson FN-13 powered turret with four .303 British Browning machine guns in the extreme tail and a pair of manually-operated .303 set on either side of the fuselage, firing from ports just below and behind the wings.

[15] Much later, a twin-gun turret was to be dorsal-mounted on the upper fuselage, about level with the wing trailing edge, bringing the total defensive armament up to 16 machine guns.

[19] On the Sunderland Mk V, fuel could be dumped from retractable pipes that extended from the hull and were attached to the bomb room side of the galley aft bulkhead.

On 3 April 1940, a Sunderland operating off Norway was attacked by six German Junkers Ju 88C fighters; during the engagement, it shot one down, damaged another enough to cause it to retreat and later perform a forced landing and drove off the rest.

While the bright Leigh searchlight was rarely fitted to Sunderlands, ASV Mark II radar enabled the flying boats to attack U-boats on the surface.

Kills fell off drastically until ASV Mark III radar was introduced in early 1943, which operated in the centimetric band and used antennas mounted in blisters under the wings outboard of the floats, instead of the cluttered stickleback aerials.

In August 1943, a captured RAF airman misled the Germans by telling them that the aircraft were homing in on the signals radiated by the Metox,[27][28] and consequently U-boat commanders were instructed to turn them off.

As a countermeasure to the increased defensive armament of the U-boats, the Australians fitted their aircraft in the field with an additional four .303s (7.7 mm) in fixed mounts in the nose, allowing the pilot to add fire while diving on the submarine before bomb release.

The addition of single .50 inch (12.7 mm) flexibly mounted M2 Browning machine guns in the beam hatches behind and above the wing trailing edge also became common.

The type's capacity to defend itself was demonstrated in particular by an air battle over the Bay of Biscay on 2 June 1943, when eight Junkers Ju 88Cs attacked a single Sunderland Mk III of No.

Two Ju 88s made simultaneous passes at EJ134 from both sides, scoring hits and disabling one engine, while the pilots fought fires and took the Sunderland through corkscrew manoeuvres.

When the Havelsee did freeze over the Sunderland's role was taken by freight-converted Handley Page Halifaxes with salt being carried in panniers fitted under the fuselage to avoid the corrosion problem.

Although the wing loading was much higher than that of any previous Royal Air Force flying boat, a new flap system kept the takeoff run to a reasonable length.

As the U-boats began to use Metox passive receivers the ASV Mk II radar gave away the presence of aircraft and the number of sightings diminished drastically.

Attacking in the dark was a problem that was solved by carrying one inch (25.4 mm), electrically initiated flares and dropping them out of the rear chute of the aircraft as it got close to the surface vessel.

In particular, a Twin Wasp Sunderland could stay airborne if two engines were knocked out on the same wing while, in similar circumstances, a standard Mark III would steadily lose altitude.

In the immediate postwar period, these were used by New Zealand's National Airways Corporation to link South Pacific Islands in the "Coral Route" before TEAL Short Sandringhams took over after 1947.

This wreck site is awkward to dive due to its proximity to the Fawley Refinery, Solent shipping lane, Calshot RNLI station and public slipway.