Special Aviation Project #1 of the United States Army Air Forces, set in motion in January of 1942, came about from a chance remark. Apparently, Hap Arnold (Commander of the USAAF) overheard someone state (or may have been casually told) that it was possible to launch a medium bomber from an aircraft carrier. This idea, coupled with a perceived need for the United States to change the narrative of the Pacific War from 'we were sneak attacked' to 'we're on the offensive' led to a proposal and later plan to bomb Tokyo using the technique mentioned.
To lead this audacious (and dangerous) operation, the USAAF turned to Lt. Col. James H. Doolittle. Doolittle had been at the forefront of aviation technology during the interwar period, working on early instrument flying and navigation methods as well as earning three major aviation trophies in competition. In 1929, he made the first completely 'blind' (what we would call IFR) flight, taking off, flying a circuit and landing at an airport in thick fog, entirely on instruments. A crash project was undertaken to make the mission possible.
The Navy aircraft carrier U.S.S. Hornet was selected for the mission, but her crew wasn't told what they were about - the captain of the ship only learned of his target right around the time the sixteen bombers were being lifted onto the deck of his ship. The plan had originally been to have the bombers take off and then return to the carrier, but it swiftly became evident that while takeoff might be possible, landing was a whole different story. Although it might have been possible to land a bomber, there was no real way to retrieve all of them as the aircraft were too large to permit recovery operations on the carrier deck. The plan was changed; the aircraft would bomb targets in Japan and then continue on to the mainland. Originally, the plan was to fly to airfields near Vladivostok - some 600 miles to the north. However, negotiations with the Soviet government were unsuccessful, and the plan was changed again - the aircraft were to fly to airfields in China. These were some 1000 to 1200 miles past Tokyo, however, meaning even more effort had to be put into extending the aircraft range.
The aircraft selected was the B-25 Mitchell, a two-engine medium bomber that had been built in large quantities and would eventually see service in every theater of the war. It provided the best compromise between performance and size - small enough to take off from the Hornet, but large enough to carry enough fuel and bomb load to make the flight worth it. Auxiliary fuel tanks were installed all over the airplane - the lower turret was removed and replaced with a tank; extra inflatable tanks were placed in the bomb bay, and tanks were plumbed into the cabin area. Spare cans of gasoline were even carried wherever they could be wedged in, and the lower turret tank was provided with a resealable cap to allow the crew to empty the cans into that tank as the flight went on before ditching them overboard.
Fuel systems weren't the only modifications made. The rear machine guns were removed, both to save weight and to save the rear crew member who would be needed to fire them. False guns of wood were emplaced in the turrets to deter any attacking aircraft from approaching from the rear - the .50 caliber guns in the B-25 were very effective, and it was thought (correctly, as Col. Doolittle confirmed in his after-action report) that the fake guns would induce attackers to attempt side approaches, more difficult and less effective. Since the mission was to take place under radio silence, the 230 lb. radio sets were removed entirely. The various pyrotechnics carried (flares for landing, generally) were also removed, with only a pair left for potential night landings.
The B-25 was normally equipped with the Norden bombsight, a highly-secret mechanical computer based system which allowed accurate bombing from high altitude, even through wind. Since the Doolittle raid was to undertake low-level bombing, that large and heavy device was removed as well (which also made those responsible for keeping it from falling into enemy hands breathe easier) and a simple $2 sight was designed and installed in all aircraft, connected to the pilot's turn indicator in the cockpit above so that the bombardier could instruct the pilot to turn without relying on voice communications. Since the original destination was to have been Vladivostock, anti-ice gear was installed (and remained) on all planes.
The aircraft and aircrews were obtained from the 17th Bombardment Group, which flew B-25s operationally. Volunteers were called for, told only that the mission was extremely hazardous and would be of great benefit to the war effort; more than enough stepped forward. Twenty-four complete air crews were sent to Wagner Field in Florida (now part of Eglin Air Force Base), where a Naval aviator was brought on board to teach them the trick of carrier takeoffs. Using painted lines to simulate the carrier deck, the crews practiced taking off in less than 300 feet - the space which would be available to the first (frontmost) airplane on the carrier. Night flying over the Gulf of Mexico helped train the crews in the exacting navigation required for over water flights at night, and bombing practice was done using incendiaries and high explosive bombs. Special unitary bombs, with additional incendiary effect, were built for the raid, and several planes (those targeting urban areas rather than industrial ones) were armed with incendiary cluster bombs.
Two planes were damaged during training; one crashed immediately after takeoff, and one had its nose gear fail at the end of the training period. With no time to replace them, the crews were ordered to San Francisco to board their ship. Eventually, sixteen aircraft were placed on board; one was to be used as a demonstrator, taking off a day out of port to show the other aircrews what their takeoffs should look like. Doolittle and the Navy, however, decided to just use all sixteen for the raid itself.
Originally, the plan was to take off some 600 miles east of Tokyo in the evening, bomb the targets at night, and then arrive in China in the morning. However, the battle group (Hornet, escorted by the carrier U.S.S. Enterprise and other escort vessels) ran into Japanese picket ships a thousand miles from Japan, and despite sinking and evading them as they went, they continued to run into them. The mission was launched some 825 miles from Tokyo, during the early part of the afternoon. Col. Doolittle flew the first aircraft off the deck, and the rest followed; all successfully left the Hornet. After takeoff, each airplane looped around to fly directly back over the ship from stern to bow; the ship's current magnetic course was displayed on the rear of the 'island' in large numbers, allowing the crews to align their compasses.
The raid was a success. With only two or three exceptions, the aircraft reached their targets and dropped their ordnance. Although the actual damage was negligible in terms of the Japanese war effort, the psychological effect was immense. One aircraft was prevented from reaching its targets by fighters, and chased westwards towards China. Nearly all aircraft made it at least as far as the China Sea, landing either in China or near the coast. The aircrews managed, in most cases, to avoid capture; however, one aircraft's crew was in fact taken and several members executed, with others remaining in Japanese hands through 1945. One aircrew turned to Vladivostok due to unexpectedly high fuel consumption, knowing they wouldn't make China; they landed safely, and were interned by the Soviet government. Doolittle himself made it back to the U.S. All members of the Raid were commended and decorated; Doolittle and several others were promoted as well.
Sources
(IN 5 20/30)