The U.S. Army Air Forces' precision strategic bombing campaign against German industries was only possible during the day. At first, this was not seen as an issue; the Forces' Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers were the most heavily armed aircraft of the time. Close formations of them were planned, creating a crossfire of .50 caliber machine-guns that would fend off the enemy with no need for a fighter escort. They remained convinced of this strategy in spite of continued warnings from the RAF that this would not be the case.