Here I will post some of the less known aircraft of Japanese empire of WWII.
All info comes from Wiki, so it might be not all 100% correct
Kawasaki Ki-64 "Rob"
The Kawasaki Ki-64 (Allied code name: Rob) was a one-off prototype of an experimental heavy, single seat, fighter. It had two unusual design features. First; it had two Kawasaki Ha-40 engines in tandem; one in the aircraft nose, the other behind the cockpit, both being connected by a drive shaft. This combination (called the Kawasaki Ha-201) drove two, three-bladed, contra-rotating propellors. The second feature was the use of the wing surface as a radiator for the water-cooled engines. The aircraft first flew in December 1943. During the fifth flight, the rear engine caught fire; and while the aircraft made an emergency landing, it was damaged. The aircraft was subsequently abandoned in mid-1944 in favour of more promising projects. The airframe survived the war, and parts of the unique cooling system were sent to Wright Field for examination.
Kawasaki Ki-88
Faced with delays in development of the Kawasaki Ki-64 fighter, the Imperial Japanese Army was open in 1942 to alternative fighter designs that might reach combat units more quickly. Kawasaki proposed the Ki-88, a design inspired by the Bell P-39 Airacobra fighter then in service with the United States Army Air Forces.
Kawasaki began design work on the Ki-88 in August 1942. The Ki-88 was to have a 1,117-kW (1,500-hp) Kawasaki Ha-140 engine behind its cockpit, driving a tractor propeller through an extension shaft. It was to mount a 37-mm cannon in its propeller shaft and two 20-mm cannon in its lower nose.
When design work had progressed far enough to allow it, Kawasaki built a full-scale mock-up of the Ki-88, which bore a strong resemblance to the P-39. After inspection of the mockup, the Japanese calculated a maximum speed for the aircraft of 600 km/hr (373 mph) at an altitude of 6,000 m (19,685 feet). This was only slightly faster than the Kawasaki Ki-61 Hien fighter, which already was in production. As a result, Kawasaki discontinued design work on the Ki-88 less than a year after beginning it.
Kawanishi Baika
Nazi Germany supplied the Japanese with a great deal of technical data, including details of the Argus As 014 pulse jet engine. However, it remains debatable whether that technical data included examples of the V1, let alone the Fieseler Fi 103R (Reichenberg). It is noted that wartime U.S. intelligence reports stated that the Japanese knew of the V-1 by October 1943, and one report stated that Japan received one example in November 1944. The reports also suggested that the Japanese were very interested in the V-1 air-launching techniques as used by the Germans. One U.S. Army Air Force document from 1946 shows the Baika as being a copy of the Reichenberg. Finally, the cargo manifest of the Japanese submarine I-29 lists a single V-1 fuselage as being included in a shipment of equipment. Nevertheless, some contemporary Japanese historians debate on what, if any, V-1 and Fi 103R data the Japanese actually received and whether the Baika may simply have been an independent design.
The only tangible outcome of the Axis cooperation, however, was the construction of prototypes of the Maru Ka10 pulse-jet engine which was to power the Baika.
Nakajima G5N "Shinzan/Liz"
The Nakajima G5N Shinzan originated due to the Imperial Japanese Navy's interest in developing a long-range attack bomber capable of carrying heavy loads of bombs or torpedoes a minimum distance of 3,000 nmi (5,600 km; 3,500 mi). To meet this requirement, it became apparent a four-engine lay-out would be necessary. As Japanese aircraft manufacturers lacked experience in building such large complex aircraft, the Navy was forced to search for a suitable existing foreign-made model upon which to base the new design. It settled on the American Douglas DC-4E airliner. In 1939 the sole prototype of this airliner (previously rejected by American airline companies) was purchased by Nippon Koku K.K. (Japan Airlines Co) and clandestinely handed over to the Nakajima Aircraft Company for dismantling and inspection.
The design that emerged from this study was for an all-metal mid-wing monoplane with fabric-covered control surfaces and powered by four 1,870 hp Nakajima NK7A Mamori 11 air-cooled radial engines driving four-bladed propellers. Notable features included a long ventral bomb-bay, glazed nose and twin tailfins replacing the DC-4E's distinctive triple rudder. The DC-4E's retractable tricycle undercarriage was retained, as well as the original wing form and powerplant arrangement. Defensive armament comprised one 20mm Type 99 Model 1 cannon each in a power-operated dorsal and tail turret plus single-mount hand-operated 7.7mm Type 97 machine guns in the nose, ventral and waist positions.
The first prototype G5N1 made its maiden flight on 14:35 8 April 1941.[1] Overall performance proved disappointingly poor however, due to a combination of excessive weight, the unreliability of the Mamori engines and the complexity of the design. Only three more prototypes were completed. In an attempt to salvage the project, two additional airframes were fitted with 1,530 hp Mitsubishi MK4B 12 "Kasei" engines and redesignated G5N2s. Although the Mitsubishi engines were more reliable than the original Mamori 11s, the aircraft was now even more hopelessly underpowered and further development of the type was halted.