When the U.S. entered World War II in December 1941 after the Japanese attack on Pearl Harbor, LeMay was a major in the United States Army Air Forces (he had been a first lieutenant as recently as 1940), and the commander of a newly created B-17 Flying Fortress unit, the 305th Bomb Group. He took this unit to England in October 1942 as part of the Eighth Air Force, and led it in combat until May 1943, notably helping to develop the combat box formation. In September 1943, he became the first commander of the newly formed 3rd Air Division. He personally led several dangerous missions, including the Regensburg section of the Schweinfurt–Regensburg mission of August 17, 1943. In that mission, he led 146 B-17s to Regensburg, Germany, beyond the range of escorting fighters, and, after bombing, continued on to bases in North Africa, losing 24 bombers in the process.