Yamamoto's main goal was to destroy America's carrier forces, which he saw as the main threat to the Pacific campaign. This concern was increased by the Doolittle Raid on 18 April 1942. In this raid, 16 US Army Air Forces B-25 Mitchell bombers launched from bombed targets in Tokyo and several other Japanese cities. The raid, while militarily unimportant, showed that American bombers could reach Japanese territory. This and other successful raids by American carriers showed that they were still a threat.