The combat box was a tactical formation used by heavy (strategic) bombers of the U.S. Army Air Forces during World War II. The combat box was also referred to as a "staggered formation". Its defensive purpose was in massing the firepower of the bombers' guns, while offensively it concentrated the release of bombs on a target. 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. Creation of the concept is credited to Colonel Curtis E. LeMay, commander of the 305th Bombardment Group in England. However the Eighth Air Force had been experimenting with different tactical formations since its first bombing mission on 17 August 1942, several of which were also known as "boxes." LeMay's group did create the "javelin down" combat box in December 1942, and that formation became the basis for the numerous variations of combat boxes that followed. The practice of referring to a concentrated formation as a "box" was the result of diagramming formations in plan, profile and front elevation views, positioning each individual bomber in an invisible boxlike area. Evolution of Eighth Air Force formations 1942 experiments The first ten missions of the Eighth Air Force from England in August 1942 were shallow penetrations of France strongly supported by Royal Air Force Spitfire fighter escorts. B-17 Flying Fortresses flew in six-plane squadrons with two to four miles between squadrons, to avoid mid-air collisions between the inexperienced crews. Although unable to support each other, the six-plane squadrons had the virtues of simplicity and ease of control. The bombers within a squadron were stacked at three altitudes with approximately between the highest and lowest, and except for the lead pair, were not in mutually supporting elements. As missions grew larger in size, deeper in penetration, and faced increasingly effective defenders, the