During the early part of the Pacific War the Japanese Navy's standard ocean patrol flying boat was the H6K - which the Allies gave the reporting-name 'Mavis'. Though Kawanishi had a technical agreement with the British firm of Short Brothers, the H6K strongly resembled a Sikorsky S.42, and was an excellent aircraft. In all 217 of the H6K were delivered, 36 of which were transports.

The matter of a replacement for the H6K Mavis was a challenge - the Navy in 1938 producing a specification calling for 30 per cent higher speed and 50 per cent greater range.

In the H8K Kawanishi produced a design which represented the biggest single leap in the design of such aircraft in all of aviation history. It was beyond dispute the best, most advanced, flying boat in the world until years after World War Two. A notable feature of the H8K was its exceptionally powerful defensive armament.

However, the H8K's early trials were disastrous - primarily because the aircraft's great weight and narrow-beamed hull produced uncontrollable porpoising. The cure was found in adding a second step in the planing bottom, adjusting the powerful double-slotted Fowler flaps, and adding a horizon mark on the large pitot post above the bows.  The aircraft's first flight took place in late 1940, and the H8K1 went into production in August 1941.

The allies gave the H8K the reporting-name 'Emily'. In all the Kawanishi plant at Kohnan built 17 H8K1, 114 H8K2 and 36 H8K2-L(a transport version). These powerful flying boats ranged the Pacific on many audacious 24-hour missions, and proved formidable. Later versions were equipped with radar, and some aircraft were provided with retractable stabilising floats