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. (February 2011)
Prior to the Big Week, throughout 1943, the U.S. 8th Air Force had been growing in size and experience and started pressing attacks deeper into Germany. It was originally believed that the defensive firepower of the ten or more .50 caliber machine guns on the B-17 and B-24 bombers would allow them to defend themselves as long as they remained arranged into tight formations, allowing for overlapping fire. In practice this proved less successful; although the bombers did claim a fair number of German fighters, losses among the bombers were unsustainable.
The Schweinfurt-Regensburg missions are a famous example of the failure of the self-defence concept. On August 17, 1943, 230 USAAF bombers launched a mission against the ball bearing factories in Schweinfurt and another 146 against the aircraft factories in Regensburg. Of this force, 60 aircraft were lost before returning to base and another 87 had to be scrapped due to irreparable damage. The Germans claimed 27 fighters lost, serious enough, but small in comparison to the losses on the part of the US forces. The Second Raid on Schweinfurt on October 14, 1943, remembered as "Black Thursday", proved even more bloody; of the 291 aircraft on the mission, 60 were lost outright, with a further 17 damaged beyond repair. Daylight missions into Germany were cancelled in order to rebuild the forces.
The raids were extensively studied by both forces. The Germans concluded that their tactic of deploying twin-engine heavy fighter designs, with heavy armament to make them usable as bomber destroyers and serving primarily with the Zerstörergeschwader combat wings, was working well. Over the winter of 1943–44 they continued this program, adding to their heavy fighter ranks and developing heavier armaments for all of their aircraft. They also pulled almost all of their fighter forces back into Germany, as the majority of their losses were due to fighter actions over forward areas. There seemed to be no point in trying to attack bombers supported by enemy fighters. The Allied forces came to the same conclusion. Schweinfurt demonstrated that the bombers were not able to adequately protect themselves, and (contrary to earlier thinking) fighter cover needed to be extended over the entire mission. Luckily for the US, the P-38 Lightning, and later the P-51 Mustang, aircraft had the range needed to escort bombers to targets deep within Germany, and were starting to arrive in quantity. Over the winter they re-equipped their fighter squadrons as Mustangs arrived and longer-range versions of existing fighters were developed.
By the early spring of 1944, both forces had laid their plans and were waiting to put them into action. The US, expecting a fighter advantage, planned missions that would demand a German response. They decided to make massive raids on the German fighter factories. If the Germans chose not to respond, they would be at risk of losing the air war without firing a shot; if they did respond, they would meet new long-range fighters in the process. The Germans needed no provocation: they were ready to meet a raid with their new forces. However, the increased weight of armaments in their fighters reduced performance, making them easy targets for the new and unexpected Mustangs.