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The H6K and H8K were Imperial Japanese Navy flying boats produced by the Kawanishi Aircraft Company and used during World War II for maritime patrol duties. The Allied reporting name for the H6K type was 'Mavis' and the Navy designation was "Type 97 Large Flying Boat", while the H8K reported as 'Emily', had the designation Type 2 Flying Boat Model 11.

H6K Type 97 Flying Boat
The H6K was designed as a result of a 8-Shi specification for a large flying boat issued by the Japanese navy in 1933. Kawanishi proposed two designs in response to this aircraft, the Type Q and Type R. Neither satisfied the Navy, and in 1934 a modified 9-Shi specification was issued. The new aircraft was to have a range of 2,500 nautical miles and a cruising speed of 120 knots. Kawanishi responded with their Type S, a parasol wing four engined flying boat. Three prototypes were constructed, each one making gradual refinements to the machine's handling both in the water and in the air, and finally fitting more powerful engines. The first of these flew on 14 July 1936 and was originally designated Navy Type 97 Flying Boat, later H6K. The Kawanishi H6K had a remarkable resemblance to the Sikorsky S-42. Both shared the same basic layout, with a parasol wing carrying four engines, similar arrangements of wing struts and similar twin vertical tails. The wings were also similar, with straight centre sections and tapering tips. However, the H6K was a much larger aircraft, eighteen feet longer, with a thirteen foot wider wingspan, and was significantly faster, with a top speed of 239mph, fifty miles faster than the S-42. The first prototype made its maiden flight on 14 July 1936, powered by four 840hp Nakajima Hikari 2 nine-cylinder air cooled radial engines. The original design had a number of flaws, most notable with the water handling and a lack of engine power. The first was solved by moving the forward step back by 50cm, and the second by installing 1,000hp Mitsubishi Kinsei 43 engines on the first, third and fourth prototypes. The modified aircraft was accepted for service as the H6K1 Navy Type 97 Flying Boat and entered production as the 'H6K2 Navy Type 97 Flying Boat Model 2'.

Into Service
The H6K made its combat debut during the war in China. When war began in the Pacific, there were four Kokutai(squadrons) that operated a total of 66 H6K4s. As well as its original long range reconnaissance role, the H6K was used for very long range bombing missions, attacking targets in the Dutch East Indies and on Rabaul. Eventually, from bases in the newly conquered Dutch East Indies, they were able to undertake missions over a large portion of Eastern and Northern Australia. This second role soon had to be abandoned in the face of more capable Allied fighters, which could take advantage of the aircraft's lack of armour or self sealing fuel tanks, but it was still able to operate as a maritime reconnaissance aircraft, operating over the vast expanses of the Pacific in areas free from Allied fighters. It would be replaced by the Kawanishi H8K 'Emily' after a production total of 217 being built.

H8K Type 97 Flying Boat Model 2
At the same time the Kawanishi H6K was going into service in 1938, the Navy ordered the development of a larger, longer-ranged patrol aircraft under the designation Navy Experimental 13-Shi Large-size Flying Boat. Specifications for the H8K called for an aircraft that could fly 5,000 miles at 210 mph and have a maximum speed of 280 mph. The result was a large, shoulder-winged design that is widely regarded as the best flying boat of the war, the Kawanishi H8K. Despite this, initial development was troublesome, with the prototype displaying terrible handling on the water. Deepening of the hull, redesigning of the planing bottom and the addition of spray strips under the nose however, rectified this.

Into Service
The Kawanishi H8K entered production in 1941 and first saw operational use on the night of 4 March 1942 in a second raid on Pearl Harbor. The improved H8K2 variant soon appeared, and its extremely heavy defensive armament earned it deep respect among Allied aircrews and as a result it was nicknamed the 'flying porcupine'. In 1945, north of the Philippines, three American submarines were sunk by radar equipped Kawanishi H8Ks. The H8K2 was an upgrade over the H8K1, having more powerful engines, slightly revised armament, and an increase in fuel capacity. This was to be the definitive variant, with 112 produced. The progressively deteriorating war situation for Japan led to a run-down in production of flying boats during 1945 in favour of fighters for home defence and later versions of the H8K were accordingly abandoned. Nevertheless this excellent aircraft saw considerable service, being flown by the 14th, 801st, 851st, 1001st, 1021st, Takuma, Toko, Yokohama and Yokosuka Chinjufu Kokutais. Altogether there were some 167 H8K's produced during the war.