The goal of Big Week was to achieve Allied air superiority over the Luftwaffe which was absolutely critical in advance of the upcoming D-Day invasion. Two tactical factors made this difficult. First, Luftwaffe fighters avoided Allied fighters and would simply ignore the fighter sweeps. Thus, the Allies could not entice the Luftwaffe fighters to engage. Second, during escort missions, Allied fighters remained in close escort formation with bombers. This tactic limited bomber casualties but it also reduced Allied pursuit and destruction of Luftwaffe fighters. Recognizing these problems, General Doolittle ordered bombing missions of key aircraft factories that the Luftwaffe could not ignore. In addition, Allied fighters were ordered to abandon the bombers with the primary goal of shooting down Luftwaffe fighters. In effect, the primary purpose of the bombing missions was to bring up the Luftwaffe and the real role of the Allied bombers was to be used and sacrificed as bait. This strategy was very effective. Freed of close bomber escort duty, Allied fighters, particularly the P-51s, decimated the Luftwaffe. German aircraft and pilot losses could not be sufficiently replaced. As a result, the Allies achieved air superiority by the time of the D-day invasion. Big Week was not primarily a bombing campaign. It was a campaign designed to kill Luftwaffe fighters.