The Aichi S1A was to be the replacement for the Nakajima J1N1-S that were being used as night fighters.
The Imperial Japanese Navy issued the 18-Shi specification for a night fighter. It was to have the following specifications:
- Speed at 29,530' / 9,000 m: 426 mph / 370 knots
- Endurance: Five hours
- Climb to 19,685' / 6,000 m: Eight minutes
- Armament: Two 30 mm cannons
- Air borne radar
Air brakes were installed in the wing and the center of the fuselage to help in not overtaking a bomber in flight.
The radar was installed in the nose.
The pilot was located in a cockpit over the wing's leading edge and the radar operator in a cockpit placed over the trailing edge. The radio operator also remotely controlled the guns in the dorsal turret.