Prior to the Big Week, throughout 1943, the US 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.