Fighter escorts were essential to the success of bombing missions. In the early stages of the war, American bombers relied on British fighters for support. These early fighters were not very helpful to the bombers, as they lacked the necessary range to provide effective cover. The P-47 Thunderbolt, for example, could only penetrate to the German border until the introduction of belly tanks, increasing their range to western Germany. The P-38 Lightning could make it to Berlin but it was the P-51 Mustang that was most effective; it was capable of round trip missions to any target in Germany.
Fighters were responsible for escorting the bombers to the initial point of their run. After the run, they would resume their duties and protect the bombers from enemy fighters. As the formations entered friendlier surroundings, the fighters were allowed to engage "targets of opportunity." The ground crews would anxiously be awaiting the returning squadrons. Flares were shot from the planes to signal that they had wounded crewmembers on board. Once the planes landed, the ground crew worked furiously to get them ready for their next flight.
Interesting Fact; The word "flak" comes from the German word Fliegerabwehrkanonen, roughly meaning "flying defense cannons".