The tight formations flown by USAAF heavy bombers allowed their defensive heavy machine guns to provide mutual cover to one another, and such a combat box was an extremely dangerous environment for a fighter aircraft to fly through, with dozens of heavy machine guns aimed at attacking Luftwaffe fighters from almost every conceivable direction. This led to numerous efforts to develop weapons that could attack the bombers outside the nominal 1,000 yards (910 m) effective range of their defensive guns.