In January 1934, the Imperial Japanese Navy had a requirement for a two-seat reconnaissance seaplane to be operated from its J-3 type submarines, and placed an order with Watanabe Ironworks for design and development of an aircraft to meet this requirement.

The E9W was a two-seat single-engine twin-float unequal-span seaplane designed to be easily dismantled for hangar stowage on a submarine, capable of being reassembled in two minutes 30 seconds and disassembled in one minute 30 seconds. It was armed with a 7.7 mm (0.303 in) machine gun operated by the observer.

The first of three prototypes flying in February 1935.

Following successful testing of one of the prototypes on the submarine I-5, an order for a production batch of 32 aircraft, designated E9W1, was placed. It was also built by Nakajima as the E9N1.

The aircraft entered service in 1938 with the Imperial Japanese Navy Air Service as the Navy Type 96 Small Reconnaissance Seaplane with the last being delivered in 1940. Although it was in the process of being replaced by the Yokosuka E14Y monoplane, it was still in front line service at the time of the Japanese attack on Pearl Harbor, remaining in service until July 1942, being used to direct their parent submarines onto Chinese ships attempting to pass the Japanese blockade of the South China Sea. The E9W1 was given the reporting name Slim in 1942 by the Allies of World War II.

E9W1
Engine: 1 × Hitachi Tempu II, 224 kW (300 hp)
Wingspan: 9.91 m (32 ft 9½ in)
Wing area: 23.51 sq.m (252.95 sq.ft)
Length: 8.00 m (26 ft 3 in)
Height: 3.71 m (12 ft 2 in)
Empty weight: 882 kg (1,940 lb)
Gross weight: 1,253 kg (2,756 lb)
Maximum speed: 232 km/h (144 mph)
Cruising speed: 148 km/h (92 mph)
Range: 731 km (454 miles)
Endurance: 4.9 hours
Service ceiling: 6,740 m (22,100 ft)
Crew: 2 (pilot, observer)
Armament: 1 x 7.7mm (0.303in) machine gun