Heavy bombers flew in formation because, that was the most effective tactics against the enemy flak and fighter defense. In a survey in 1943, it was discovered that just over half of the bombers shot down by the Germans during attacks on target in Europe, were those that to leave the main formation. Geting into formation took up a lot of time and effort, used up a good percentage of valuable fuel and increased the possibility of pilot fatigue. The procedure for getting a Group into correct formation also took up an hour's flying time but mission planners at US Air Force knew that the procedure was essential. Between 1942 and 1945 a number of ideal formations were worked out and modified.
A bomb from one B-17 hits the stabilizer of the bomber below it. A deadly illustration of the importance of correct and accurate Group formation during a bombin run. This machine was part of the 94th BG. It was said in the official comment "no single parachute could be seen".