Beginning with B-17F-30-BO, B-17F-20-DL, and B-17F-20-VE, external bomb racks were fitted under the inner wings for the carriage of two 4000-pounds bombs. This brought the maximum short-range bombload to 17,600 pounds. Under certain conditions, eight 1600-pound bombs could be carried internally and two 4000-pound bombs externally, raising the total load to 20,800 pounds. However, with such a load the effective range was quite small and all maneuvers were severely restricted. Consequently, external bombs were only rarely carried by the B-17F. Although all subsequent models had lugs and controls for their attachment, the under wing racks were not installed at the factory.





Also see - LINK - http://www.axis-and-allies-paintworks.com/e107_plugins/forum/forum_viewtopic.php?422
Regards Duggy