There is certainly no question about the 20th's place in history including the operation of the B-29's including "Boxcar" and the "Enola Gay". The 20th was formed in the United States on 4 April, 1944 and didnt enter combat until later that year for about a year of actual combat.
However, we should not sell the efforts of the 5th Air Force short, especially considering their active service in the early war years. The 5th was originaly formed in the Phillipines on 16 August, 1941 as a part of the Phillipines Defense Forces. If memory serves me correctly it was the only numbered Air Force during WWII to be formed overseas. It was subsequently re-designated the "5th" Air Force in February 1942. The 5th flew a number of the less glamorous or less well heralded type aircraft, including the P-39 and P-40 fighters, the A20, A24 and B-26 attack/bomber aircraft and toward the end of the war Convair's B-32 Dominator. Kenney's "Flying Bucaneers", like many units early on during the war, both Army and Navy, were instrumental in developing new tactics, techniques and field modifications in order to accomplish their missions, many of which were copied by subsequent units as the war unfolded.
The 5th was essentially McArthur's Air Force and was critical in the defense of Australia and New Guinea. While most of us certainly agree that the Battle of Midway was the key turning point in the war in the Pacific, halting the Japanese advance on Port Morseby , New Guniea, first by sea in the Battle of the Coral Sea and subsequently by the air and land campaign from spring 1942 through July 1944, effectively ended the Japanese conquest of the South Pacific especially for Australia. While the 5th's role in the Battle of the Coral Sea was minimal, it was absolutely critical for the longer haul, land and air battle for New Guinea. The 5th Air Force ultimmately slogged its way up the western portions of the Pacific including the reconquest of the Phillipines until it finaly wound up on Okinawa at the end of the war. It was in continous combat from Pearl Harbor to Tokyo Bay, 1941-1945, or 3 1/2 years of actual combat. It also produced a large number of Aces including Richard Bong.
The 5th was also, for all practical purposes, under the command and or the control of one man throughout the entire war. That man was MG George C. Kenny who assumed command of the 5th Air Force in August 1942. He directly commanded the 5th until the summer of 1944 when the 5th and 13th Air Forces were combined under an umbrella organization, known as the Far East Air Force. Both the 5th and the 13th retained their own organization and comand structures, the overall planning for both uints was handed over to the higher sole command. General Kenny was given command of the Far East Air Force.
There are a number of exceptional books and websites on the 5th in general as well as many of its subordinate units.