Inspired by the Heinkel He 119, Yokosuka began to design an aircraft of a similar layout, known as the Y-40, in 1943. Headed by Commander Shiro Otsuki, the aircraft project was a pressurized, two-seat, unarmed, high-speed, reconnaissance aircraft of all-metal construction that featured tricycle retractable gear. The Japanese Navy decided to take advantage of this work, and issued an 18-Shi specification built around the Y-40. The design was approved, and the Y-40 officially became known as the R2Y1 Keiun (Beautiful Cloud). The construction of two prototypes was ordered.

Commissioned for the Imperial Japanese Navy after the R1Y design was cancelled due to its disappointing performance estimates, the R2Y used coupled engines driving a single propeller and also featured a tricycle undercarriage. The Yokosuka R2Y Keiun (景雲 – “Cirrus Cloud”) was a prototype reconnaissance aircraft.

The Keiun was powered by two 60-degree, inverted V-12 Aichi Atsuta 30 series engines, licensed-built versions of the Daimler-Benz DB 601. The engines were coupled together by a common gear reduction in a similar fashion as the DB 606. The resulting 24-cylinder power unit was known as the Aichi [Ha-70]. With a 5.91 in (150 mm) bore and 6.30 in (160 mm) stroke, the engine displaced 4,141 cu in (67.8 L) and was installed behind the cockpit and above the wings. The Aichi [Ha-70] engine was to be turbocharged and rated at 3,400 hp (2,535 kW) for takeoff and 3,000 hp (2,237 kW) at 26,247 ft (8,000 m). Without the turbocharger, the engine was rated at 3,100 hp (2,312 kW) for takeoff and 3,060 hp (2,282 kW) at 9,843 ft (3,000 m). The engine drove a 12.47 ft (3.8 m), six-blade propeller via a 12.8 ft (3.9 m) long extension shaft that ran under the cockpit. Engine cooling was achieved by radiators under the fuselage and inlets for oil coolers in the wing roots. A ventral air scoop was located behind the engine to provide induction air for the turbocharger and air for the intercooler. Speculation suggests the first scoop on the side of the aircraft provided cooling air for the engine’s internal exhaust baffling, the second, larger scoop provided induction air for the normally aspirated Aichi [Ha-70] engine installed in the prototype, and the final two ports were for the engine’s exhaust.

The pilot sat under a raised bubble-style canopy that was toward the extreme front of the aircraft. The radio operator/navigator occupied an area in the fuselage just behind and a little below the pilot.

By the fall of 1944, the direction of the war had changed, and Japan no longer needed a high-speed reconnaissance aircraft. The R2Y1 Keiun was all but cancelled when the design team suggested the aircraft could easily be made into a fast attack bomber. In addition, the Aichi Ha-70 power plant would be discarded, and one 2,910 lb (1,320 kg) thrust Mitsubishi Ne 330 jet engine would be installed under each wing. A fuel tank would be installed in the space made available by the removal of the piston engine. The bomber version would carry a single 1,764lb bomb under the fuselage, and carry cannons in the nose. This jet-powered attack bomber had an estimated top speed of 495 mph (797 km/h). The Japanese Navy decided to accept the modified design. Yokosuka were given permission to produce one R2Y1 piston-engined prototype to test out the aerodynamics of the design, while also working on the jet-powered R2Y2 Keiun Kai. It did not enter construction before the end of the war.

The decision was made to finish the nearly completed R2Y1 airframe and use it as a flight demonstrator to assess the flying characteristics of the aircraft. With pressurization, the turbocharger, and the intercooler omitted, the R2Y1 prototype was completed in April 1945 and transferred to Kisarazu Air Field for tests. Ground tests revealed that the aircraft suffered from nose-wheel shimmy and engine overheating.

Adjustments were made to overcome the issues, and the Keiun took to the air on 29 May 1945 (date varies by source and is often cited as 8 May 1945), piloted by Lt. Commander Kitajima. The flight proved to be very short because the engine quickly overheated, and a fire broke out in the engine bay. Lt. Commander Kitajima quickly returned to the field, and the R2Y1 suffered surprisingly little damage.

On 31 May during a ground run to test revised cooling, the engine was mistakenly run at high power for too long and overheated. The engine was removed from the aircraft to repair the damage. The R2Y1 sat awaiting repair for some time before it was destroyed by Japanese Naval personnel to prevent its capture by American forces (some say it was destroyed in an Allied bombing raid).

Because of the end of the War, the second R2Y1 prototype was never completed nor was the design work for the R2Y2.

Three were built, but only one was completed.

R2Y1
Engine: 2 x Aichi-10 Ha-70, 2550kW (3,400 hp / 3,100hp at 9,845ft)
Propeller: 6-bladed constant-speed metal
Wingspan: 14.0 m / 45 ft 11 in
Length: 13.05 m / 43 ft 10 in
Height: 4.24 m / 13 ft 11 in
Wing area: 34.0 sq.m / 365.97 sq ft
Max take-off weight: 8100-9400 kg / 17858 – 20724 lb
Empty weight: 6015 kg / 13261 lb
Fuel capacity: 1,555 l (411 US gal; 342 imp gal)
Max. speed: 715 km/h / 444 mph at 10,000 m (32,808 ft)
Cruise speed: 460 km/h / 286 mph at 4,000 m (13,123 ft)
Landing speed: 166 km/h (103 mph; 90 kn)
Ferry range: 3,611 km (2,244 mi, 1,950 nmi)
Service ceiling: 11,700 m (38,400 ft)
Time to altitude: 10,000 m (32,808 ft) in 21 minutes
Crew: 2

R2Y2
Engine: Two Ne-330 axial-flow turbojets
Power: 2,910lb thrust each
Crew: 2 (pilot and radio operator/ navigator)Span:
Armament: Forward firing cannon
Bomb load: One 1,764lb bomb