Initially formations were created in keeping with the pre-war Air Corps doctrine that massed bombers could attack and destroy targets in daylight without fighter escort, relying on interlocking fire from their defensive machine guns, almost exclusively the "light barrel" Browning AN/M2 .50-calibre (12.7 mm) gun. However the use of high altitudes by USAAF bombers resulted in factors that demanded a tighter bomb pattern and the combat box continued in use even after the advent of fighter escort – and especially starting in the spring of 1944 over Europe, with USAAF fighters flying far ahead of the combat boxes in air supremacy mode instead against the Luftwaffe's fighters – largely mitigated the threat of fighter interception.[1]