This list comes from Norm Avery's Book, B-25 Mitchell: Magnificent Medium. If there are any errors of any type, please let me know as soon as possible. I am trying to link each group/squadron that has a website here. If you know of one that I do not have linked here, please email me with that information.
Fifth Air Force
Squadrons: 8th 13th 89th 90th
Squadrons: 69th 70th (Transfered out in early in the war) 71st 405th 822nd 823rd
345th Bomb Group (To view other sites about the 345th, go to the links section)
Squadrons: 498th 499th 500th 501st
Squadrons: 2nd 33rd 408th
Seventh Air Force
Squadrons: 47th 48th 396th 820th
Ninth Air Force
12th Bomb Group (To 12th AF Aug 1943)
Squadrons: 81st 82nd 83rd 434th
340th Bomb Group (To 12th AF Aug 1943)
Squadrons: 486th 487th 488th 489th
Tenth Air Force
12th Bomb Group (From 12th AF Feb 1944)
341st Bomb Group (To 1 lth AF Nov 1943)
Squadrons: 1 lth 22nd 490th 491st
1st Air Commando Group (Another interesting link)
(Operated a section of 12 B-25H's for several months in 1944)
Eleventh Air Force
28th Composite Group
Squadrons: 73rd 77th 406th
Twelfth Air Force
12th Bomb Group (From 9th AF Aug 1943)
310th Bomb Group (History of 310th also included in links to 321st and 340th BG's.)
Squadrons: 379th 380th 381st 428th
Squadrons: 445th 446th 447th 448th
340th Bomb Group (From 9th AF Aug 1943)
Thirteenth Air Force
Squadrons: 69th 70th 75th 100th 390th
Fourteenth Air Force
1st Bomb Group (Chinese American Composite Wing)
Squadrons: 1st 2nd 3rd 4th
341st Bomb Group (From 10th AF Nov. 1944- Less 490th BS)