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.