The Kawanishi H8K 'Emily' was Japan's preeminent flying-boat, a superb design which was also considered by many to be the finest flying-boat of World War II, far superior to anything in the US or British arsenals. Successor to the H6K 'Mavis', the H8K entered service early in the war and served up to the very end although it would take some time before it fully superseded its predecessor in front-line service. It had incredible range, an endurance of up to 25 hours, and was able to carry a considerable bomb load; best of all (and surprising for a Japanese aircraft), it was sufficiently protected with defensive armament and partially self-sealing fuel tanks to be quite difficult to shoot down, a quality which was to prove indispensable as Japan's air superiority was slowly eroded. For this reason, H8Ks were successful in every Pacific front that the Japanese were deployed in up to 1945 and were used for reconnaissance, transport, and torpedo bombing missions, often equipped with ASV radar. So advanced was the H8K that it would take many years after the war for another flying boat to equal it in performance and technology.

The initial H8K1 prototype flew for the first time on 31 December 1940 but was plagued with handling problems in water. Once overcome, the principal production variant became known as the H8K2 which replaced the Kasei 12 engine with a Kasei 22, it became the standard Japanese flying boat after 1943 and was someitimes fitted with ASV radar as well as being used for reconnaissance, bombing and torpedo attacks. A transport version was designated H8K2-L Seika (Clear Sky) which could carry 64 passengers while the H8K3 and H8K4 had extra armor protection and retractable wing-tip floats but were unfortunately never produced.