In World War I not a single American-built plane flew in action against the enemy. In the 1920's most theoreticians considered the nascent American air arm to be a mere toy, a plaything which had no real relevance in modern war. Billy Mitchell's warnings to the contrary went unheeded until the mid-1930's, when weapons development and training of pilots belatedly began. But the early American warplanes like the P-38 and P-40 were found wanting when the war began. When the Pearl Harbor disaster struck, America was still painfully unready for the war in the air, but technical developments, like the B-17 bomber, were already being built. The story of the rise of the American Air Force is one of the classic stories of World War II. By mid-war the US Army Air Force, the 8th Air Force operating out of Britain, and the 15th operating from North Africa and Sicily, fought alongside the RAF and won respect around the world. The Naval Air Force helped to win the Battle of Midway and began to push the Japanese back toward the home islands. American warplanes became the best that fought on either side: the P-51 Mustang was perhaps the best fighter aircraft in the war, and the B-17 and its successors, the Liberator and the Superfortress, became the most effective bombers in the war.