The prototype Nakajima B5N equipped with an 800 hp 9-cylinder, radial Nakajima Hikari-2 engine, flew for the first time as early as January 1937, demonstrating maximum speed of 370 km/hr and exceeding all 10-Si design specifications. Series production of this airplane equipped with a 840 hp Hikari-3 engine and designated Type 97 Naval Carrier-Based Strike Bomber Model 1 (B5N1) started up at the end of 1937. Despite a number of simplifications made in its design in the process of flight testing, B5N1 still remained an advanced machine for its time: it had an all-metal, monoplane, smooth-surfaced airframe; retractable landing gear, louvered flaps and a hydraulically powered arrester hook, as well as a three-blade constant speed propeller and integral large-volume fuel tanks located in the center wing section. Thanks to replaceable ordnance attachment gear, it could carry either a 450-mm 800-kg Type 91 torpedo (designed in 1931) or ordnance of up to three 250-kg high-explosive bombs. The crew of three sat one behind the other in a single cockpit. A navigator-bomber, sitting between the pilot and a gunner-radioman, had a bombsight in a center section hatch for horizontal-flight bombing. During takeoff and landing, the pilot's seat was elevated to improve field of view. Fuselage was made very compact so that the plane would fit in an aircraft carrier lift. The large wings were folded using manually operated winches with the left folding on top of the right.

Land-based B6N2s. Note radar antennas along the leading edge and at the tail.

The crews of carrier-based B5N1s began to practice torpedo attacks early in 1938, and from the end of that year these aircraft successfully supported Japanese troops first in China and later in Indochina. As flight crews accumulated combat experience and the aircraft became more effective only minor modifications were made. It was only at the end of 1939 when the B5N1s fighting in China began to suffer heavy losses from modern fighter aircraft that a major modernization became a necessity. In December 1939 the Japanese started testing a new Model 12 (B5N2, American code name “Kate”) of the aircraft and by the end of 1941 it completely replaced the older Model 11. The engine was replaced with a 14-cylinder radial Nakajima Sakae-11 rated at 1000 hp, which increased operational ceiling and field of view but only slightly improved the rate of climb and speed. Fuel tanks remained unprotected, and weaponry still consisted of only one Type 92 7.7-mm machine gun mounted in the gunner-radioman's cockpit. Some of the aircraft were equipped with ground surveillance radars or magnetic submarine detectors and depth bomb carrying stations. By the end of 1943, the fleet of B5N2s had been heavily decimated, and this aircraft's useful life virtually ended in June 1944 when the last squadron participated in the Battle of the Philippine Sea. Low speed, inadequate defensive weapons, low survivability, American air dominance, and increased effectiveness of shipboard and land-based anti-aircraft artillery all contributed to its demise.

A number of scholars believe that the Japanese leadership, elated with their initial military successes, failed or were unable to reequip their aircraft carriers with modern attack aircraft. However, it was back in 1938 that the Japanese issued the 13-Si design specification for the development of a modern dive bomber to replace D3A1 (this effort resulted in the Yokosuka D4Y bomber) and in 1939 the 14-Si specification for the development of a new torpedo bomber (B6N). In both cases technical problems with propeller drive train and engines forced the Japanese to start production of modified D3A2 and B5N2 aircraft instead. In 1941 Nakajima concentrated all its efforts on the B6N and transferred production of B5N2 to Aichi plants and the 11-th Naval Aviation Arsenal, which until the middle of 1943 made another 480 of this aircraft.

D4Y1 has just dropped its bombs. Note open bomb bay doors.	Damaged D4Y3 coming out of a dive. Note extended air brake flaps and smoke trail.

Naval Carrier-Based Strike Bomber Tenzan Model 11 (Nakajima B6N1 “Sacred Mountain”, allied name “Jill”) was fielded in the spring of 1943 after two years devoted to perfecting the design. Almost immediately afterwards the aircraft engine was replaced once more: production of Nakajima's Momaru-11 was terminated in favor of a more widely used Mitsubishi Kasei-25 with the same rating. The Tenzan Model 12 (B6N2) became the primary Japanese torpedo plane (the number of aircraft produced by the end of the war amounted to 1133) but was unable to score any significant successes. Improved aerodynamics and 80% higher rated engine as compared to the one installed on the B5N2 increased the new aircraft's cruise speed by 100 km/hr. However, armor protection for the pilot and self-sealing tanks were rejected (it would have reduced their useful volume by almost a third) and the bomb load even decreased. One small-caliber machine gun mounted in a ventral hatch was added to the self-defense weapons. By 1944 the Americans enjoyed absolute air superiority over the Japanese both in number of aircraft and performance. Whether used as a torpedo bomber or as a kamikaze plane, the B6N2 utterly failed to stand up to the American F6F Hellcat carrier-based fighters and Marine Corps F4U Corsairs.

The service career of the Yokosuka D4Y Susei (“Comet”) dive bomber, which by 1944 replaced all D3As, was more successful. D4Y1 was the only land-based aircraft in WWII history that managed to sink an aircraft carrier: On October 24, 1944 in the Battle of Leyte Gulf (the Philippine Islands) one of several Japanese dive bombers that broke through sank the American light aircraft carrier Princeton with a 500-kg bomb. On March 19, 1945 60 km off the Japanese coast, another Japanese D4Y3 damaged the heavy American aircraft carrier Franklin when it scored two direct hits with 250-kg bombs. In general, however, the D4Y's potential capabilities outweighed its actual achievements.

Profiles of D4Y scanned from Ref. 1

The Yokosuka D4Y or “Judy” was the fastest and the most beautiful single-engine dive bomber of WWII. It was superior to the American SBD-5 Dauntless and SB2C Helldiver in all respects except bomb load, self-defense weapons and, most importantly, absence of protective lining for its fuel tanks. This last disadvantage, aggravated by insufficient pilot experience (almost all experienced pilots were dead) and lack of reliable air cover (Zero fighters and their pilots were no match for American machines and pilots), turned the D4Y debut into a catastrophe. It took place on June 19, 1944 during a battle between aircraft carrier groups in the Philippine Sea off the Mariana Islands. Later the Americans called the encounter "the Great Mariana Turkey Shoot". Flying his F6F Hellcat with a damaged engine, Lieutenant Alexander Vrachou, with 12 air victories already to his credit, shot down six Japanese D4Y1s, using only about 60 12.7-mm rounds on each in a single fight over the aircraft carrier Lexington. On that day Japan lost 297 aircraft together with 9 aircraft carriers, including more than 100 land-based aircraft and the latest Tyho aircraft carrier to be sunk by an American submarine. The next day American carrier-based aircraft sank two Japanese tankers and seriously damaged five more aircraft carriers, a tanker and a battleship. Americans paid for this victory with only 26 of their aircraft and minor damages to two of their battleships.

D4Y1-C reconnaissance plane with external tanks.	D4Y1 dive bombers at an island airfield.

The history of the Yokosuka D4Y dive bomber began at the end of 1938 with the issue at to the 1st Naval Technical Aviation Arsenal in Yokosuka of the 13-Si design specification for the development of a fast carrier-based dive bomber similar to the German Henkel He-118-V4 dive bomber that the Japanese fleet had bought and tested under the designation DXHe-1. The enthusiasm of the Japanese naval leaders for the plane was not seriously dampened even after the DXHe-1 disintegrated in the air: the aircraft was very compact and very fast, with perfect aerodynamics and excellent handling characteristics. After working for two years on the project, Yokosuka designers made a completely new machine, retaining from the original DXHe-1 only the perfect aerodynamics and in-line water-cooled engine - the 600th series Daimler-Benz. The two-seat carrier-based bomber was slightly larger than the Zero fighter (so that it was able to do without folding wings) but carried half as much bomb load and fuel as the D3A1 aircraft it was meant to replace. A big 500-kg bomb was carried internally in a large fuselage bomb bay – a deciding factor in the selection of the mid-wing design. The main landing gear was fully retractable into the wings towards the fuselage. At hard points beyond the landing gear the aircraft could carry a load of additional 30-kg bombs or fuel tanks which increased its range to twice that of the D3A bomber. The entire length of the trailing edge was occupied by large flaps and ailerons with air brakes in front. Self-defense weapons remained the same as in the D3A – two synchronous and one turret-mounted (magazine-fed) small caliber machine gun.

Left side of D4Y2 cockpit (“Koku-Fan” magazine, 1980, N 4)	D4Y4 instrument panel (“Koku-Fan”, 1975, N 11)	Right side of D4Y2 cockpit (“Koku-Fan” magazine, 1980, N 4)

The first prototype (equipped with the DB600G engine) rose into the air in Yokosuka in December of 1940. It demonstrated performance characteristics comparable to the latest Mitsubishi A6M2 Zero fighter. The preparation for production started at the plants in Aichi and Nagoya, but very soon universal enthusiasm for the plane became muted because its wings revealed a tendency to flutter while diving, threatening the aircraft with instant destruction. However, since the machine was much faster than all existing naval reconnaissance planes it was decided to convert the initial production batch: bomb attachment gear was removed and photo reconnaissance equipment installed instead. By that time a new Aichi AE1A “Atsuta”-12 engine (licensed version of DB601A) rated at 1200 hp became available. In this configuration in the spring of 1942 the aircraft was accepted into service as “Type 2 Carrier-Based Scout Model 11” (D4Y1-C) and was used as such in small numbers until the end of the war.