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JAPANESE
The Japanese produced the best flying boat of the war: excellent performance, long range and good defensive and offensive armament.
The Kawanishi H8K Type 2, a high performance, four-engined bomber, reconnaissance and transport aircraft code named "Emily" by the Allies. Type 2 flying boats even raided Honolulu, Hawaii in March 1942. Unfortunately they were often used as scouts for Japanese carrier striking forces. However in that role they suffered greatly at the hands of radar directed carrier-borne fighters as they approached U.S. task forces.
The H8K-2 incorporated armor protection for the crew and protected fuel tanks in the wings and fuselage. Defensive armament included single 20mm cannon in the nose turret, dorsal turret, tail turret and (port and starboard) beam blisters. These were augmented by five .303 machine guns firing from hull hatches. Offensive weapons included two torpedoes or 2205 pounds of bombs and depth charges. Performance was excellent, the best of any of the big WW II flying boats. Top speed was almost 300 mph at around 20,000' and the service ceiling was over 28,000'. After the Japanese developed radar, surface search sets were fitted to H8K patrol bombers.
There was a transport version of the H8K which were produced by modifying the existing aircraft. Most of the defensive armament, including the side blisters and dorsal turret, were removed as were the fuselage fuel tanks to increase its cargo volume. The result of these modifications was that the H8K-2L could transport up to 62 combat troops.
As an island nation with few natural resources almost totally dependent upon their merchant shipping they none-the-less failed to protect their merchant shipping. American submarines sank about half of the entire Japanese merchant fleet with little interference from the air. These aircraft could have performed an invaluable service protecting convoys from submarine attack. The traditional Japanese offensive spirit cost them dearly during the war
GERMANY
While it did not reach production or operational service and only one prototype was ever built that was destroyed while tied to its dock at Schaal Lake in September 1944 by three P-51 fighters. You have got to stand in awe at ship builder Blohm & Voss's Bv-238. This huge, six-engined flying boat weighed 120,800 pounds and had a 220,460 pound maximum takeoff weight. A top speed of 264mph and a 4,113 mile range
Then we have the Bv-222 Viking which first flew in September 1940 and was carrying supplies and personnel between Hamburg and Norway in 1941. By the Fall of 1941, the Bv 222 was operating in the Mediterranean theatre, ferrying supplies to the Afrika Corps. As a troop transport, the Bv 222 could transport up to 92 passengers or 72 wounded men on stretchers. The Bv-222 became the largest aircraft to shoot down an enemy airplane during the war, when a Bv 222 (V4) shot down a PB4Y (B-24) patrol plane.
Essentially a custom built aircraft since only 13 Bv 222's ever were built and thus specifications were highly variable. From 1941 onward they were armed with a 13mm machine gun in a forward top turret located behind the control cabin another 13mm machine gun in an aft dorsal turret, four lateral firing 8mm machine guns (two/side) in waist mounts and an 8mm nose gun. In aircraft V2 this arrangement was augmented by turrets mounted on the upper surface of each wing aft of the outboard engines, each carrying a pair of 13mm machine guns. There was an even later armament scheme consisting of three 20mm cannons (one each in the forward top turret and the wing turrets, the aft dorsal turret having been deleted) plus five 13mm machine guns (one in the nose and two/side).
The Bv 222 could haul its crew of 11 and 92 troops and their supplies on its two decks. Most freight and the majority of troops were carried on the lower (cargo) deck. Fuel was carried in a huge tubular wing spar and a passageway in the wings gave access to the engines during flight an also to the wing mounted machine gun turrets that were fitted to V2 and later aircraft.
Four Bv-222 survived the war and were captured by the allies. None survive today.
AMERICAN
Consolidated Vultee's PB2Y Coronado flying boat was a four-engined patrol bomber, transport and flying naval ambulance first ordered by the U.S. Navy in 1936 as the successor to the obsolescent PBY Catalina. Both types served throughout the war. The PB2Y-3 was produced from late 1941 until 1944 and incorporated self-sealing fuel tanks and improved armor protection for the crew. In 1944, production shifted to the PB2Y-5 using Pratt & Whitney R-1830-92 radial engines to save weight and increase load capacity. The change did degrade high altitude performance. However, PB2Y's did not usually operate at high altitude.
The transport versions could carry 44 passengers or 16,000 pounds of cargo. The PB2Y-5H flying ambulance version could accommodate up to 25 stretcher cases. Only one Coronado survives today, the others having been scraped after the war.
It is also important to mention the Martin JRM-1 Mars transport with its 145,000 pound takeoff weight. However only seven were completed since the first production Mars did not enter service until the summer of 1945. The Coronado served throughout the war and was very competitive with the more famous Sunderland flying boat, except in range. The earlier, smaller and less expensive PBY Catalina handily outranged the Coronado, which was the key to the Catalina's longevity.
Saunders-Roe made some, as did Supermarine.Did the British have any other big flying boats besides the Sunderland ? I couldn't find any except the Empire but those were civilian transports, albeit quite impressive ones. The closest the Italians had was the CANT Z. 506 but I put that in a different category since it is a twin-float.
I should have mentioned that the He59 should be on the list because it was vital in the sea-rescue of any downed pilots during it's Seenotdienst service
Never fear Grant, I brought up the unjustly overlooked and often maligned Walrus in an earlier post!...And where is the Supermarine Walrus?