The C6N originated from a 1942 Imperial Japanese Navy specification for a carrier-based reconnaissance plane with a top speed of 350 knots (650 km/h) at 6,000 m and range of 2,500 Nautical miles (4,960 km). Nakajima's initial proposal, designated N-50, was for a craft with two 1,000 hp engines housed in tandem in the fuselage, driving two propellers mounted on the wings. With the development of the 2,000 hp class Nakajima Homare engine though, this configuration was abandoned and Nakajima decided on a more conventional single-engine layout. However, the Homare's output turned out to be less than initially expected, so the design had to be optimized in other areas. The resulting aircraft was designed around a long and extremely narrow cylindrical fuselage, just large enough in diameter to accommodate the engine. The crew of three sat in tandem under a single canopy, while equipment was similarly arranged in a line along the fuselage. The C6N's low mounted laminar flow wing housed fuel tanks and was fitted with both Fowler and slit flaps and leading edge slats to lower the aircraft's landing speed to ease use aboard aircraft carriers. Like Nakajima's earlier B6N "Tenzan" torpedo bomber, the rudder was angled slightly forward to enable tighter packing on aircraft carriers.
The first flight was on 15 May 1943, with the prototype demonstrating a speed of 639 km/h (345 kt, 397 mph). Performance of the Homare engine was disappointing, especially power at altitude, and a series of 18 further prototypes and pre-production aircraft were built, before the Sauin was finally ordered into production in February 1944.
Although designed for carrier use, by the time it entered service in September 1944, there were few carriers left for it to operate from, so most were used from land bases. Its speed was exemplified by a famous telegraph sent after a successful mission: "No Grummans can catch us." ("我に追いつくグラマンなし"). The top speed of the Grumman F6F Hellcat was indeed of the same level, so overtaking a Saiun was out of the question.
A total of 463 aircraft were produced. A single prototype of a turbocharged development mounting a 4-blade propeller was built, this was called the C6N2 Saiun-kai. A night-fighter version C6N1-S with oblique-firing (Schräge Musik configuration) single 30 mm (or dual 20 mm) cannon and a torpedo carrying C6N1-B were also developed. The C6N1-B developed by Nakajima was not needed after Japan's aircraft carriers were destroyed. As Allied bombers came within reach of the Japanese home islands, there became a need for a first class night fighter. This led Nakajima to develop the C6N1-S by removing the observer and replacing him with two 20mm cannons. The C6N1-S's effectiveness was hampered by the lack of air-to-air radar, although it was fast enough to enjoy almost complete immunity from interception by Allied fighters.
Despite its speed and performance, on 15 August 1945, a C6N1 was the last aircraft to be shot down in World War II. Just five minutes later, the war was over and all Japanese aircraft were grounded.
Three prototypes and sixteen supplementary prototypes produced, four-blade propeller, latter batch were equipped three-blade propeller, mounted Nakajima NK9K-L Homare 22 engine, #6 was mounted Nakajima NK9H Homare 21 engine. Renamed Test production Saiun (試製彩雲, Shisei Saiun) in July 1943.
C6N1 Saiun Model 11 (彩雲11型, Saiun 11-gata)
General production model. Three-blade propeller, mounted Nakajima NK9H Homare 21 engine.
Temporary rebuilt two-seat night-fighter version, this was not a regulation naval aircraft. Development code C6N1-S was not discovered in the IJN official documents. One model with a singular 30 mm Type 5 cannon and at least five models with x2 20 mm Type 99-1 cannon were converted from standard C6N1 models. One surviving example of the x2 20mm cannon variant is stored in the Paul E. Garber Preservation, Restoration, and Storage Facility.
C6N2 Test production Saiun Kai/Saiun Model 12 (試製彩雲改/彩雲12型, Shisei Saiun Kai/Saiun 12-gata)
Fitted with four-blade propeller, 1,980-hp (1476-kW) Nakajima NK9K-L Homare 24-Ru turbocharged engine. Two prototypes were converted from regular C6N1 models in February 1945.
C6N3 Test production Saiun Kai 1 (試製彩雲改1, Shisei Saiun Kai 1)
High-altitude night-fighter version of the C6N2. Dual 20 mm cannons were installed. Only a project.
C6N4 Test production Saiun Kai 2 (試製彩雲改2, Shisei Saiun Kai 2)
Fitted 2,200-hp Mitsubishi MK9K-L Ha 43-11 Ru turbocharged engine, one prototype was converted from C6N1, incomplete.
C6N5 Test production Saiun Kai 3 (試製彩雲改3, Shisei Saiun Kai 3)
Proposed torpedo-bomber version. Only a project.
C6N6 Test production Saiun Kai 4 (試製彩雲改4, Shisei Saiun Kai 4)