'Conceal' was the US bombing attack, otherwise known as the 'Special Aviation Project No. 1', on Tokyo and other targets in the Japanese home islands (18 April 1942).

This so-called 'Doolittle raid' was the first air raid by the US forces against the Japanese home island of Honshu, was intended to demonstrate that the Japanese home islands were vulnerable to Allied air attack, and provided an opportunity for limited retaliation for the Japanese 'Ai' attack on Pearl Harbor on 7 December 1941.

Immediately after the Pearl Harbor raid, the US Navy had been ordered to counterattack the Japanese as quickly as possible. The problem the service faced, however, what where and when to strike with its depleted assets, now centred on surface elements such as its aircraft carriers, cruisers and destroyers without major capital ship support, and sub-surface elements in the form of its submarine arm. So far as the latter was concerned, it soon became apparent that while its boats were for the most part excellent and well-suited to operations over long distances in the Pacific, the inter-war combination of slow promotion rates and an overly cautious tactical doctrine meant that most of the boats were commanded by men past the age to accept new thinking and therefore wedded to the idea of sonar-informed torpedo attacks from considerable depth.

Yet two or more years of British experience against the U-boats allowed them to inform the Americans that the Germans had developed and gained great success with attacks from just under the surface using the periscope as the primary sensor and night attacks on the surface. Moreover, as events were soon to reveal, there were major problems with the US Navy’s torpedoes, which tended to run too deep and suffer detonator failure.

On 11 January 1942, some 500 miles (800 km) south of Oahu, Commander Hiroshi Inada decided, one hour after sunset, that he had the US fleet carrier Lexington in the sights of his submarine I-6, and launched salvo of Type 93 torpedoes at very long range. In fact Inada’s target was Saratoga, Lexington's sister ship, on patrol as part of Vice Admiral Herbert F. Leary’s Task Force 14: one torpedo struck the carrier on the port side well below the waterline, killing six men and injuring five, and flooding three compartments. It was fortunate for the Americans that the carrier had been built on the armoured hull of an incomplete battle-cruiser in 1925. Even so, it took three hours to get Saratoga back onto an even keel by counter-flooding, but then the ship managed to raise 16 kt to return to Pearl Harbor. TF14 was then broken up, Leary was sent to command the ANZAC forces in the southern Pacific, and Saratoga steamed to California for repair and modernisation.

Vice Admiral Wilson Brown, commanding TF11 from Lexington, was then forced to abandon an air raid on Japanese-occupied Wake island on 23 January, when Lieutenant Commander Togami’s I-172 sank Neches, TF11’s oiler, in the same area. Having covered a troop convoy to reinforce Samoa, to the north-east of Fiji, Vice Admiral William F. Halsey’s TF8 and Rear Admiral Frank J. Fletcher’s TF17 (centred on the fleet carriers Enterprise and Yorktown respectively) were ordered by Admiral Chester W. Nimitz to attack the Japanese targets in the Marshall islands group of the western central Pacific, from which the Americans believed that any Japanese attack on the Samoa was likely to be launched. The US task forces departed Samoa on 31 January and attacked half a dozen atolls, including Kwajalein, with cruiser and destroyer gun bombardments as well as air raids by level bombers, dive-bombers and fighters. The Japanese tried to counterattack Enterprise with land-based bombers from island airfields. Very little damage was done by either side, but the news of the first surface attacks and the wildly exaggerated damage claims were invaluable to service as well as civilian morale.

Halsey’s task force then raided Wake island on 24 February, again with little measurable effect. Halsey next launched a carrierborne air raid on Marcus island, 1,000 miles (1600 km) to the south-east of Tokyo and 2,000 miles (3200 km) to the west of Hawaii, on 4 March.

Late in February, Brown and Lexington were temporarily attached to the ANZAC command to attack the Japanese strategic base at Rabaul, New Britain, in the Bismarck archipelago off the north-eastern coast of the great island of New Guinea. Three Japanese patrol aircraft sighted the US ships approaching from the east on the morning of 20 February and some 350 miles (565 km) from their target. One of the aircraft managed to get away, and in the afternoon the first major air battle of the war between Japanese and US maritime aircraft took place. With the element of surprise now lost, and it being clear that Rabaul was strongly defended, Brown cancelled the raid.

He was given Fletcher’s Yorktown group for a second attempt in March. The target was changed on 8 March in the light of news that the Japanese were attempting to establish themselves, in 'Sr', at Lae and Salamaua on the Huon Gulf of North-West New Guinea. Brown adopted the clever tactic of attacking from a point to the south of New Guinea, over the Owen Stanley mountains, and from only 50 miles (80 km) offshore. The raid of 104 US carrierborne aircraft began soon after dawn on 10 March. For the loss of one aeroplane the attackers sank three Japanese ships, a small result that only briefly incommoded the Japanese operations on the ground.

These first carrierborne sorties by the US Navy were tactical pinpricks but had psychologically important consequences. For the Americans, they boosted morale and provided some much-needed combat practice. They did not inconvenience the Japanese unduly, but caused loss of face to commanders who had boasted that the Americans would be unable to strike back for months. But there was one more US air attack during this period of greatest Japanese success in the first six months of the Pacific War, and this reflected the fact that Admiral Ernest J. King, the commander-in-chief of the US Navy, and his staff were very conscious of the pressure of public opinion for positive action of a major nature.

The raid had its roots in a desire by President Franklin D. Roosevelt, expressed to the Joint Chiefs-of-Staff at a meeting in the White House on 21 December 1941, that Japan be bombed as soon as possible. Early in 1942 King had ordered Captain Donald B. Duncan, his air operations officer, to examine the possibility of a bombing attack on Tokyo in specific retaliation for Pearl Harbor. This could be achieved only by warplanes operating from an aircraft carrier, but the US Navy had no carrierborne aircraft capable of undertaking the task, which required longer ranges and heavier payloads than those that current carrierborne aircraft could provide. The raid would have to be launched at least 500 miles (800 km) from the Japanese capital, outside the 300-mile (485-km) radius of Japanese coastal air patrols and the picket line of guard boats known to be on station 500 miles (800 km) out.

Duncan took three days to draft a plan for a raid by two carriers, one carrying the bombers and the other providing protective cover at sea. The new fleet carrier Hornet would embark the raiders and Enterprise would provide the cover. Halsey would command this specially created TF16. The attack plan was undertaken by Captain Francis Low, who was in fact a submariner rather than naval aviator.

On 10 January 1942 Low reported to King that he believed twin-engined bombers of the US Army Air Forces could be launched from an aircraft carrier: this followed Low’s observation of several such aircraft at a naval airfield in Norfolk, Virginia, where the runway had been painted with the outline of a carrier deck for landing practice. The requirements for the aircraft type to be used included a cruising range of 2,400 miles (3860 km) with a 2,000-lb (907-kg) bomb load, and the selection process soon resulted in the selection of the North American B-25B Mitchell twin-engined medium over three other twin-engined types, the Douglas B-18 Bolo, Douglas B-23 Dragon and Martin B-26 Marauder: the B-26 had questionable take-off characteristics from a carrier deck, and the B-23’s span was nearly 50% greater than that of the B-25, reducing the number that could be taken aboard a carrier and posing risks to the ship’s island; and the B-18, one of the final two types considered, was rejected for the same reason.

King therefore enlisted the support of Lieutenant General Henry H. Arnold, chief of the USAAF. After planning had suggested that the B-25 was the type best meeting the requirements for the operation, two were loaded aboard the carrier Hornet at Norfolk, Virginia, and subsequently flown off the deck without difficulty on 3 February 1942. The raid was immediately approved, and Lieutenant Colonel William C. Mills’s 17^th Bombardment Group (Medium) was selected to provide the pool of crews from which volunteers would be recruited. In the spring of 1942, the 17^th Bombardment Group had been the first group to receive the B-25, and all four of its squadrons had received the type by September: thus it was not only the first medium bomb group of the USAAF but also had on its strength the most experienced B-25 crews.

The 17^th Bombardment group, then flying anti-submarine patrols along the west coast of the continental USA from Pendleton, Oregon, was immediately moved to Lexington County Army Air Base, Columbia, South Carolina, for the ostensible purpose of flying similar patrols off the eastern coast. On 9 February the group officially arrived at Columbia AAB, where its combat crews were offered the opportunity to volunteer for an 'extremely hazardous' but unspecified mission. Initial planning called for 20 aircraft to fly the mission, and 24 of the group’s B-25B bombers were diverted to the Mid-Continent Airlines modification centre in Minneapolis, Minnesota, for modifications including the removal of the ventral gun turret, installation of de-icers and anti-icers, mounting of steel blast plates on the fuselage around the upper turret, removal of the liaison radio set, installation of three additional fuel tanks and support mounts in the weapons bay, crawl way and lower turret area to increase fuel capacity from 646 to 1,141 US gal (2445 to 4319 litres), installation of dummy gun barrels in the tail cone, replacement of the Norden bomb sight with an improvised 'Mark Twain' sight devised by pilot Captain C. Ross Greening, and mountings in two aircraft for cameras to record the results of the bombing.

The 24 crews selected picked up the modified bombers in Minneapolis and flew them to Eglin Field, Florida, beginning on 1 March 1942. There the crews received intensive training for three weeks in simulated carrier deck take-offs, low-level and night flying, low-altitude bombing, and over-water navigation. One aeroplane was heavily damaged in a take-off accident and another taken off the mission because of a nose wheel shimmy that could not be repaired in time.

On 25 March the remaining 22 B-25 bombers took off from Eglin for McClellan Field, California, which they reached on 27 March for final modifications at the Sacramento Air Depot. Some 16 B-25 aircraft were then flown to Alameda, California, on 31 March: 15 would comprise the mission force and the final machine, by last-minute agreement with the US Navy, would be squeezed onto the deck to be flown off shortly after departure from San Francisco with the intention that it be used provide feedback to the USAAF pilots about take-off factors, but in the event was made part of the mission force.

On 1 April the modified bombers, their five-man USAAF crews and maintenance personnel (71 officers and 130 enlisted men) were loaded onto Hornet at Alameda. Each aeroplane was equipped to carry four special 500-lb (227-kg) bombs: three of these were high-explosive munitions, and one was a bundle of incendiaries. The last were long tubes, wrapped together in order to be carried in the bomb bay, but designed to separate and scatter over a wide area after release.

To reduce weight and so increase range, the bombers' defensive armament was reduced: each aeroplane took-off with two 0.5-in (12.7-mm) machine guns in an upper turret and one 0.3-in (7.62-mm) machine gun in the nose. Two wooden, simulated gun barrels mounted in the tail cones were intended to discourage Japanese air attacks from that direction, and were cited afterward by Doolittle as being particularly effective.

The aircraft were clustered closely and tied down on Hornet's flight deck in the order of their expected launch.

The 'Conceal' plan was fraught with technical and operational problems. The aircraft were too large to be struck down to the hangar deck and would therefore have to remain on the flightdeck, and thus be vulnerable to the adverse conditions of wind and weather throughout the approach to Japan. While it was just possible to launch the aircraft from a carrier, landing them on a quite probably pitching and/or rolling deck out of the question. The aircraft were therefore not return to Hornet but push on to China and hope to land at an airfield, 1,100 miles (1770 km) from Tokyo, controlled by Generalissimo Chiang Kai-shek’s nationalist forces, which could not be told the reason.

The commander of the raid itself was Lieutenant Colonel James H. Doolittle, who was given one month to train his men as the aircraft were modified. As noted above, the 16 aircraft then flew to San Francisco Bay for loading onto Hornet on 1 April. Formally known as Task Force 18, the carrier departed the port of Alameda at 10.00 on 2 April, and at this time comprised the carrier Enterprise, carrying 27 fighters, 18 torpedo-bombers and 36 scouting dive-bombers, one heavy cruiser, one light cruiser, four destroyers and one oiler. Enterprise's fighters and scout aircraft would provide protection for the entire task force in the event of a Japanese air attack, since Hornet's fighters were stowed below decks to allow the B-25 bombers to use the flight deck.

TF18 rendezvoused on 13 April with Halsey’s Task Force 16 in the mid-Pacific Ocean to the north of Hawaii, where the submarine Thresher was the beacon. The combined force of two carriers, the heavy cruisers Northampton, Salt Lake City and Vincennes, the light cruiser Nashville, the destroyers Balch, Benham, Ellet, Fanning, Grayson, Gwinn, Meredith and Monssen, and the fleet oilers Cimarron and Sabine then proceeded in radio silence.

On the afternoon of 17 April, the oilers refuelled the task force, then withdrew with the destroyers to the east, while the carriers and cruisers dashed west at 20 kt toward their intended launch point in the waters to the east of Japan.

But in the early hours of 18 April, more than 700 miles (1125 km) out from the planned launch area, the radar of the US warships detected two Japanese ships on picket duty much farther out than expected, and the US force changed course to avoid them and the possibility of detection. At 07.38 on the morning of 18 April, however, while the ships were still about 650 miles (1045 km) from Japan, the task force was sighted by the Japanese picket boat No. 23 Nitto Maru which radioed an attack warning to Japan before being fatally damaged by gunfire from the cruiser Nashville.

Halsey had planned that the bombers be launched in the afternoon to arrive over Japan in the dark, but now decided to launch the mission immediately, some 10 hours early and 170 miles (275 km) farther from Japan than planned, which meant that the bombers would arrive over Japan in daylight. After respotting to allow for engine start and run-ups, Doolittle’s aircraft each had 467 ft (142 m) of take-off distance.

Despite the fact that none of the B-25 pilots, including Doolittle, had any previous experience in a carrier take-off, all 16 aircraft took off safely between 08.20 and 09.19. (The 16^th B-25 had been included only as a reserve, intended to fly along as an observation and photographic platform, but when the mission was compromised Doolittle made a command decision to make operational use of this reserve machine).

The Japanese had been partly alerted by signals traffic analysis that US carriers were at sea, and Admiral Isoroku Yamamoto, commander-in-chief of the Combined Fleet, therefore ordered the pickets to be especially vigilant and reinforced the very light air defences around Tokyo. An anti-aircraft exercise took place over the city that morning. As noted above, No. 23 Nitto Maru had reported the approach of three carriers, and the Japanese calculated that the single-engined aircraft they were carrying would have to be launched from as close as 200 miles (320 km), implying an attack after dark by the time the carriers were close enough to launch their aircraft. But by launching twin-engined aircraft from so far out, Halsey achieved tactical surprise: the Mitchell bombers needed less than two hours to make up the distance it would have taken the ships more than 12 hours to cover, so no sooner was the exercise over than Tokyo was confronted with the real thing, at 12.00, at least eight hours earlier than expected.

The bombers flew toward Japan in groups of two to four aircraft before changing to a line ahead arrangement at wave-top height to avoid detection. The aircraft began to arrive over Japan at about 12.00 local time, and bombed 10 military and industrial targets in Tokyo, two in Yokohama, and one each in Yokosuka, Nagoya, Kobe and Osaka. Although some of the bombers encountered light anti-aircraft fire and a few fighters over Japan, no bomber was shot down. Only the B-25 of Lieutenant Richard O. Joyce received any battle damage, in the form of minor hits from anti-aircraft fire, while the fourth machine, flown by Lieutenant Everett W. Holstrom, jettisoned its bombs before reaching its target when it came under attack by fighters after its gun turret malfunctioned. Some 15 of the 16 aircraft then proceeded to the south-west along the south coast of Japan and across the East China Sea toward eastern China, where several fields in Chekiang province were supposed to be ready to guide them in using homing beacons, then recover and refuel them for continuing on to Chungking.

One B-25, extremely low on fuel, headed instead for the closer USSR. The raiders faced several unforeseen challenges during their flight to China: night was approaching, the aircraft were running short of fuel, and the weather was deteriorating quickly. None would have reached China at all except for a fortuitous tail wind as they came away from the target, and this increased their ground speed by 25 kt for seven hours. As a result of these problems, the crews realised they would probably not be able to reach their intended bases in China, leaving them the option of either bailing out over eastern China or crash-landing along the Chinese coast.

Fifteen aircraft reached the Chinese coast after 13 hours of flight and were crash-landed or had their crews bail out; the crew which reached the USSR landed 40 miles (65 km) beyond Vladivostok, where its aeroplane was confiscated and the crew interned until they managed to escape through Iran in 1943.

This was the longest combat mission ever flown by the B-25 Mitchell medium bomber, averaging some 2,250 miles (3620 km). Doolittle and his crew, after safely parachuting into China, received assistance from Chinese soldiers and civilians as well as John Birch, a US missionary in China. As did the others who participated in the mission, Doolittle had to bail out but fortunately landed in a heap of manure in a rice paddy near Chuchow. Doolittle thought that the raid had been a failure because all the aircraft had been lost, and that he would be court-martialled upon his return.

Following the Doolittle Raid, most of the B-25 crews which came down in China eventually made it to safety with the help of Chinese civilians and soldiers. The Chinese people who helped them, however, paid dearly for sheltering the Americans. The Japanese army began the 'Campaign for Zhejiang-Jiangxi' to intimidate the Chinese from helping downed US airmen. The Japanese killed an estimated 250,000 civilians while searching for Doolittle’s men.

The 10 crew members of two aircraft were initially listed merely as missing, but on 15 August the USA learned from the Swiss consulate general in Shanghai that eight of the missing crew members were prisoners of the Japanese at police headquarters in that city (two crewmen had died in the crash landing of their aircraft). On 19 October 1942 the Japanese announced that they had tried the eight men and sentenced them to death, but that several of them (in fact five) had received commutation of their sentences to life imprisonment. No names or details were included in the broadcast. Of the five men imprisoned, one died.

On the basis of the picket boat reports, Vice Admiral Chuichi Nagumo’s 1^st Air Fleet, comprising five fleet carriers, four battleships, two heavy cruisers, one light cruiser and nine destroyers, currently to the east of Formosa on its return to Japan from the 'C' raid into the Indian Ocean, was put on the trail of the US attacking force, but on 22 April had to abandon the attempted interception as what was now clearly a lost cause. Likewise, the submarines I-21, I-22, I-24, I-27, I-28 and I-29, which were to the Bonin islands group en route from Japan to Truk in the Caroline islands group, and also the 3^rd Submarine Flotilla's I-8, I-74, I-75, I-68, I-69, I-71 and I-72, were unable to catch TF16, which had withdrawn at high speed to return to Pearl Harbor.

Correctly deducing that the raid was, at least for the time being, a single effort, the Japanese staff did not plan a fully optimised air-defence system and Japan was therefore unable to cope effectively when still larger and more numerous US bombers began to raid Japan in 1944. The most immediate consequence of 'Conceal' for the Japanese was the temporary diversion of four army fighter groups which urgently needed elsewhere.

'Conceal' was the only occasion on which USAAF bombers were launched from a US Navy aircraft carrier on a combat mission. After the raid, Doolittle told his crew that he believed the loss of all 16 aircraft, coupled with the relatively minor damage the aircraft had inflicted on their targets, had rendered the attack a failure. But in fact the raid bolstered US morale to such an extent that Doolittle was awarded the Medal of Honor and upgraded two grades to brigadier general.

By comparison with the devastating raids by Boeing B-29 Superfortress four-engined heavy bombers later in the war, the Doolittle raid did little material damage: eight primary and five secondary targets were attacked, and the Japanese reported that the two aircraft whose crews were captured had also struck their targets. But the raid had a major strategic impact, although this was not understood at the time, inasmuch as it led the Japanese to recall some fighter units to the home islands for defence.

The Japanese did not understand how US aircraft could attack from such a distance, and assumed that the USA had developed a new and extremely long-range aeroplane. This assumption that the home islands were vulnerable to air attack strengthened Yamamoto’s resolve to extend Japan’s planned outer defence perimeter, and this led directly to the 'Mi' action against Midway island, with disastrous results for the Japanese navy, as well as the parallel 'Al', 'Aob' and 'Aq' undertakings against the Aleutian islands group.