Answer: North American B-25 Mitchell
Following the repudiation of the Washington and London Naval Treaties, the US Navy put in place plans for a third Yorktown-class aircraft carrier. USS Hornet was commissioned on 20 October 1941 as the final US aircraft carrier commissioned before the United States entered the Second World War. The ship was initially commissioned at Norfolk and was undertaking shakedown and carrier qualifications when Pearl Harbor was attacked less than two months after the ship entered service. As a result, it was determined that Hornet would be required as soon as possible to reinforce the Pacific Fleet, which led to her training programme being accelerated.
In February 1942, a pair of B-25 Mitchell medium bombers were loaded aboard Hornet before departing on a training mission off the coast of Virginia. While at sea, the two land-based aircraft were then successfully launched from Hornet's deck. No explanation was forthcoming to the ship's company, who then sailed for the Pacific on 4 March. Arriving in San Francisco on 20 March, Hornet's own aircraft were stored in the ship's hanger, and 16 B-25s were loaded onto her flight deck. Hornet departed on 2 April on a secret mission to undertake a bombing mission of Japan. The so-called "Doolittle Raid", named for the commanding officer of the B-25 squadron, took place when the B-25s were successfully launched from Hornet on 18 April and dropped bombs on Tokyo, the first time since the the attack on Pearl Harbor that the US had attack the Japanese home islands.