15th AF resulted from a reorganization of Doolittle's Twelfth Air Force into the 15th with Doolittle in command, and the Ninth Air Force (9th AF) with Lewis H. Brereton in command. The new air force was activated with a strength of ninety B-24 Liberators and 210 B-17 Flying Fortresses, inherited from the Twelfth Air Force and Ninth Air Force. In December, new groups, most of which were equipped with B-24s soon started arriving from the United States. 13 new groups were added.