From September 1939, the Navy started to accept deliveries of a new «Model 22» bomber equipped with high-altitude Kinsei-45 engines (1000 hp at 4000 meters) and a completely new set of weapons. The top hemisphere was now protected by a 20-mm Type 99 cannon mounted in a turret, and additional small-caliber machine guns were mounted in the side blisters and pilot's cabin. The modifications did not help much, but despite heavy losses, by the middle of 1938, G3M became the primary Japanese bomber. When Mitsubishi switched over to production of the G4M model in February 1941, G3M2 production was transferred to Nakajima plants. In the autumn of 1941 Nakajima started production of the Model 23 G3M3 equipped with 1300-hp Kinsei-53 engines. The 1048th and final plane in this series was made in 1943.

«Model 22» and«Model 21» in the sky over Malay peninsula, January 1942. Aircraft of the «Mihoro» air group, which participated in the sinking of «Prince of Wales» and «Repulse».

By December 7, 1941, the forward located naval air groups in Indochina and Formosa numbered 204 G3M and about 150 G4M1 bombers. The first air strikes of the Pacific War were not against Pearl Harbor, as generally believed, but against Singapore, by 27 G3M2 bombers at 4:30 on the morning of December 8, 1941. During the next 14 hours the Japanese bombed not only Hawaii but also Wake Island, Thailand, the Philippines, Guam and Hong-Kong. From 12:35 to 12:45 on that memorable day 25 G3M2 and 81 G4M1 bombers destroyed 80 American aircraft stationed on the airfields of Lawson Island, including nearly half of all Boeing B-17 Flying Fortresses in the American arsenal at the time. On December 8, 1941 G3M2 and G4M1 bombers sank the pride of the British Navy, the battleship Prince of Wales and the battle cruiser Repulse, off the coast of Malaya. The «Z force» under Admiral Phillips was heading to lay waste to Japanese invasion troops on the Malay Peninsula. Phillips was only concerned about the danger of Japanese battleships, assuming that Japanese military aircraft, based in Saigon, were too far away to pose a threat. Maintaining radio silence, Phillips did not request air defense fighters from Singapore when Japanese scout planes were spotted. At 6:25, 34 G3M2 bombers, carrying two 250-kg or one 500-kg bomb each, took off, to be followed by 23 G3M2 and 26 G4M1 bombers carrying torpedoes. At about 11:15 eight bombers launched the first attack on the Repulse with one 250-kg bomb finding its target. Soon thereafter, eight G3M2 torpedo planes struck at the Prince of Wales, slamming two torpedoes into its side. Other torpedoes, launched by these planes missed, but both ships were attacked again, this time by “Betty” bombers. In this second attack, four more torpedoes struck the Prince of Wales and five the Repulse. At 12:33 the Repulse overturned, and 15 minutes later 17 newly-arrived G3M2s launched another attack against the Prince of Wales, hitting it directly with a 500-kg bomb. As a result, at 13: 20 the ship also keeled over and quickly sank. Anti-aircraft guns on the British ships managed to shoot down one "Nell" torpedo bomber and a couple of "Betty" bombers. Some of the Japanese bombers were damaged and a couple destroyed in crash landings upon return to base. Such losses were but a slight price to pay for the destruction of the main force of the British Far Eastern Fleet! But even in the first months of the war - a period of unquestionable Japanese air superiority from Wake Island to Port Darwin in Australia - G3M losses quickly mounted, especially once the Allied fighter force started to grow in numbers and became more active. Starting from 1943, these bombers were gradually withdrawn from active duty and used for training or support missions. The last G3M bombers fought in February 1944 in the Battle of the Marshall Islands.

The Japanese Navy's long-range bomber air groups were now flying only Mitsubishi G4M bombers. In September 1937, the Navy Staff had placed a contract for the development of a fast bomber capable of carrying internally an improved 1055-kg torpedo over a distance of 3700 km. After studying the requirements, Mitsubishi engineers proposed a four-engine bomber that could carry twice as much load, but Navy commanders declined the proposal, arguing that such a plane would be too ineffective in torpedo attacks. By the end of the year they finally agreed on a compromise - a twin-engine bomber with G3M range and a 420 km/hr cruise speed. Actual work on this machine started up only in April 1938 when Toshio Honyе, having returned from a training course in the United States, commissioned the development of a 3D low-drag fuselage to aerodynamicists from the 1st Naval Aviation Arsenal in Yokosuka. It's interesting to note that German engineers who saw the G4M fuselage during a tour of the Mitsubishi plant decided that it was a four-engine bomber.

G6M1 escort fighter 1940.	G4M1 «Model 11» bomber, winter 1941-1942.

The G4M1 prototype flew for the first time on October 23, 1939. The test program continued until the spring of 1940. Engineers had modified the vertical tail area several times before finally achieving excellent handling characteristics in all flight regimes. With a load of an internally transported torpedo, the aircraft could cruise at 428 km/hr; after dropping the torpedo, speed increased to 465 km/hr in horizontal flight and 520 km/hr in shallow descent. The range at a 325 km/hr cruise speed and at 4000 m altitude was more than 6000 km! But at this stage Japanese Naval commanders made another mistake. G3M bombers were suffering heavy losses in China and Staff "strategists," impressed by the high speed of the new bomber, pushed through the idea of a G4M1-based long range escort fighter with a crew of 10 and armed with six 20-mm swivel cannons and self-sealing fuel tanks. From August until November 1940 thirty G6M1 Type 1 Naval Escort Fighter planes were made, but the idea failed to live up to expectations: the increased weight and drag slowed the plane down so much that after dropping their bombs, the G3Ms left their fighter escort far behind. It was only in December 1940 that the G4M1 bomber was finally launched into production. In the same winter 13 production aircraft were accepted for army testing.

The twin-motor, all-metal cantilever midwing monoplane machine had a crew of 7-8 inside a large monocoque oval cross-section fuselage. 4780-liter fuel tanks without any protective lining were located between two wing spars: two in the center section plus one in the inner and two in the outer sections of each wing. Unprotected oil tanks formed the leading edges of the inner wings with electrically actuated flaps attached to their trailing edges. To the inner wing sections were attached twin-row 14-cylinder air-cooled Mitsubishi “Kasei”-11 engines with take-off rating of 1530 hp, equipped with twin-speed single-stage supercharges and three-blade metal “Sumitomo Hamilton” propellers. Ailerons with 15% axial aerodynamic compensation were located along the trailing edge of the outer wing sections. The main landing gear was fully retractable into forward-located pods by means of electric drive. Up to 1000 kg in bomb load or a torpedo were carried in a large internal bomb bay in the center section. In case of an 800-kg bomb or a torpedo, the bomb bay doors were partially removed and smaller ordnance was carried without installing any doors at all. 7.7-mm Type 92 machine guns (Vickers-type guns fed from 6-7 magazines with 97 rounds in each) were mounted in the nose in the navigator cockpit and three gun blisters at the top and sides. At the tail, behind a single vertical stabilizer, there was a hand operated turret equipped with a 20-mm automatic “Type 99” cannon fed from 8 magazines with 45 rounds in each. The aircraft was equipped with an autopilot, radio compass and optical bomb-sight.

On April 2, 1941 Type 1 Naval Strike Bomber Model 11 Mitsubishi G4M1 was officially entered in the Army inventory and by November, up to 25 machines were manufactured each month at an aircraft assembly plant in Nagoya. In May, the “Takao” air group began to practice torpedo attacks, and from June it started to fly combat missions in China. In one such mission a group of G4M1s used their 20-mm tail cannons to repel an attack of Chinese I-16s, shooting down two of the attackers. In the first months of the "big" war G4M1s were also highly successful against allied warships and transports as well as allied aviation sitting on the ground. By December 8, nearly half of the U.S. Asian Air Force was destroyed at air bases in the Philippines and on December 10—a black day for the British Royal Navy—G4M1 bombers burned 42 American aircraft on the ground and completely destroyed the Cavite naval base, sinking several transport ships and severely damaging a destroyer and two submarines. In this attack Japanese aircraft destroyed all American stocks of torpedoes, thus forcing the American destroyers to head for Australia and Hawaii. By the end of December up to twenty transports and tankers had been sunk off the coast of the Philippines alone. At the same time, in December and January, “Betty” bombers were pounding American airfields in Malaya and blocking shipping routes to Singapore. Thus, on January 21, 25 G3M2s and 27 G4M1s supported by the Army's Ki-21 and Ki-48 raided Singapore, reducing entire city blocks to rubble. In addition, “Betties” sank a transport ship in the port and burned 13 “Blenheims” at Tenga airfield. On January 22, G4M1 bombers destroyed another 20 British aircraft at Kalang airfield losing one of their crews to “Hurricanes”. The “Hurricanes” paid for that single victory dearly - five of them were shot down by “Zero” fighters escorting the bombers. At the end of January and beginning of February 1942, “Betty” bombers started attacking Sumatra and Java, where they met an organized resistance. So, on February 18, 21 G4Ms escorted by 15 “Zeroes” attacked the port of Surabaya, sinking a gun-boat and submarine and damaging several other ships. But 12 American P-40s intercepted the group and managed to shoot down four and severely damage another nine Japanese bombers. An attempt on February 20, 1942 of 17 G4M1s from the Genzan air group to attack the American 11th Operational Force sailing to Rabaul ended in a catastrophe for the attackers – F4F-3 Wildcats and SBD-2 Douglases from the aircraft carrier Lexington shot down 15 of the Japanese aircraft, and the two surviving planes barely managed to return to base. The ships sustained only minor damages but Japanese gunners shot down two American F4F-3 fighters. On that day, American lieutenant O'Hara added five more aircraft to his list of victories (it should have been three—two of the “Betties” were only damaged). From March 1942 fighting for New Guinea became much fiercer, and “Betties” started to pound the Australian port of Darwin, the main source of allied reinforcements. Very soon, though, they were strongly rebuffed by American P-40E Kittyhawks: in two bombing raids on April 25 and 27 “Betty” losses were as high as 30%. In July the Allies launched a counteroffensive in New Guinea, and from August 1942 they began to push the Japanese out of Solomon Islands in action that spelled the beginning of the Guadalcanal slaughter. “Betties” supported Japanese land troops, attacked allied invasion flotillas and destroyed enemy aviation on the ground. Aircraft losses steadily mounted and it was becoming increasingly difficult to find replacements—the shortage of experienced air crews was particularly acute.

The crews often burnt alive in these “flying lighters”. Even bullets from small-caliber anti-aircraft artillery could easily ignite the bombers, which with their heavy bomb loads could not evade such fire by climbing higher, as engine performance declined significantly with an increase in altitude. In April 1942, the Japanese began flight testing of the new Model 12 G4M1 bomber with high-altitude Kasei-15 engines, which increased aircraft operational ceiling to 9660 meters and maximum speed to 452 km/hr. The first deliveries of the new bomber started in November, but very soon all gains in flight characteristics were counter-balanced by weight increases: all machines starting from production No. 663 now received a carbon dioxide fire-fighting system and rubber-lined bottom in their fuel tanks. The operating range decreased despite larger 5600-liter fuel tanks. It was decided to start development of a completely new model, namely the G4M2, featuring laminar wing profile, 6200-liter fuel tanks, rotating turret on top instead of a machine gun blister armed with a 20-mm Type 91 cannon, more powerful Mitsubishi Mk.4O Kasei-21 engines, and four-blade Sumitomo-VDM propellers. Prototype testing conducted from November 1942 to February 1943 revealed that the new propellers were insufficiently reliable, so the new G4M2 Model 22 bomber launched into production in July 1943 at a plant in Nagoya was equipped with old three-blade propellers. Because of an engine deficit, the old Model 12 bomber continued to be manufactured in parallel until January 1944. All G4M2 aircraft starting from No. 275 were armed with 20-mm cannons at the sides instead of machine guns. Such a version was designated Model 22A. Model 22B received an improved Type 99 Model 2 20-mm cannon in the top turret. On May 31, 1943 a new G4M2a Model 24 prototype was completed. It differed from Model 22A in its more powerful and higher altitude MK.4T Kasei-25 engines. The Model 24С became the principal bomber of the Japanese army (600 machines manufactured). It was armed with 13-mm Type 2 machine guns in the navigator cockpit and 20-mm Type 99-2 cannons in other gun stations

G4M1 cockpit [6].	Arming a G4M1 bomber with a torpedo.

G4M2 «Model 22»	G4M2a «Model 24»

By the end of 1942 navy leaders realized that the concept of a long-range twin-motor bomber was a mistake: the aircraft most effective against enemy ships were highly-maneuverable single-engine dive-bombers and torpedo-bombers, while hard targets such as ports and air bases were best tackled by heavy and heavily armed four-engine "flying fortresses." The shift in priorities primarily reflected the fact that Japan was now fighting a defensive war, though to some extent the successes of the American B-24 Liberators in destroying the Japanese fleet in its home bases was also a factor. In 1938 the Naval Staff issued an order to Nakajima for the development of a four-engine bomber with 6500 km range. Since they had no experience in heavy bomber design, the Japanese bought an American Douglas DC.4E airliner in 1939, and by the end of the year Nakajima had assembled the first prototype of a Naval Experimental Strike Bomber Sinzan G5N1. This machine became the first Japanese aircraft with a nose wheel. The airplane was much too heavy and after manufacture of five prototypes the program was terminated.

G5N1, 1941