The Aichi S1A "Denko" was in development with the Empire of Japan during World War 2 (1939-1945) as a successor to the Nakajima J1N1 "Gekkou" - or "Gekko". The Gekko proved itself since its introduction in May of 1941 as a very capable twin-engined heavy fighter. It saw service as a long-ranged reconnaissance platform but, more importantly, as a night fighter fitted with radar and obliquely-angled (30-degrees) 20mm cannons designed to strike at the vulnerable bellies of Allied bombers.
As good as the Gekko was, the arrival of the American Boeing B-29 "Superfortress" from May of 1944 on over Japanese territories across the Pacific Theater began to outclass the early-war Nakajima design - it lacked the high-altitude performance and a more capable radar fit needed to contend with the new threat. As such thought was given to a new twin-engined heavy type with modern qualities and exceptional capabilities and this charge fell to Aichi as other, more prominent, aircraft-builders of the war were tied to other production commitments for the Empire.
The resulting design became the "S1A" carrying a crew of two and outfitted with the required mission equipment for the night-hunter role. Dimensions included an overall length of 15 meters, a wingspan of 17.5 meters, and a height of 4.6 meters. Structurally, the S1A1 was to become Japan's largest fighter of the war. Empty weight was 7,320 kg against a Maximum Take-Off Weight (MTOW) of 11,510 kg. Power was from 2 x Nakajima NK9K-S air-cooled, twin-row 18-cylinder radial piston engines developing 2,000 horsepower each and driving four-bladed propeller units.
The aircraft's general configuration was typical of Japanese heavy fighter entries seen during World War 2 . It utilized a central fuselage containing the crew of two, avionics, fuel stores, and vital mission equipment. The cockpit was seated well-forward of midships and aft of a short nosecone assembly. The nose section held the standard fixed, forward-facing armament. At midships was a framed observation dome seated ahead of a turret emplacement. The tail unit relied on a single vertical fin with low-mounted horizontal planes along the aft fuselage sides. The wing mainplanes were ahead of midships and each held an underslung engine nacelles - the pilot being given unobstructed views of both installations. The mainplanes themselves were straight in their general design with clipped wingtips. The undercarriage was a typical "tail dragger" arrangement.
Performance for the S1A in prototype form included a maximum speed of 360 miles per hour, a cruising speed of 275 miles per hour, a range out to 1,555 miles (ferry), and a service ceiling of 39,370 feet.
Armament centered on 2 x 30mm Type 5 cannons and 2 x Type 99 Model 1 cannons fitted to the nose section for considerable forward-facing firepower. The aforementioned turret emplacement along the fuselage spine fitted 2 x 20mm Type 99 Model 2 cannons offering a trainable weapon's position.
The S1A was a promising design by mid-war standards but its projected weight was beginning to suffer as more and more mission equipment was added. Additionally, the intended Nakajima radials were not presenting the required power that IJN authorities sought so thought was given to fielding S1A1 production models with Mitsubishi engines instead. On December 7th, 1944, there was also a major earthquake off the coast of Japan - the "Tonankai Earthquake" (Magnitude 8.1) - which resulted in the two ordered prototypes being heavily damaged along with their production facilities. Allied bombing raids in June and July of 1945 ended all hope for the aircraft as both prototypes were destroyed and the work was taken up again.
Japan surrendered to the Allies during August of 1945 bringing about a complete end to World War 2.