The new German tactics of using "Sturmböcke" (heavily armed Fw 190s) as bomber destroyers and Bf 109Gs to escort them in "Gefechtsverband" formations, were proving somewhat effective. The US fighters, kept in close contact with the bombers they were protecting, could not chase the attacking fighters before they were forced to turn around and return to the bombers. Lt. Gen. Jimmy Doolittle, commander of the 8th Air Force from the end of 1943, responded by initiating a breakthrough in fighter tactics by "freeing" the fighters, allowing them to fly far ahead of the heavy bomber formations in an air supremacy "fighter sweep" mode on the outward legs; then following the USAAF heavies' bomb runs, the fighters roamed far from the bomber streams and hunted down German fighters — especially the "Sturmböcke", that had limited maneuverability with their heavy underwing conformal gun pod-mount autocannons — before they could ever approach the USAAF bombers. Though the change was unpopular with the bomber crews, its effects were immediate and extremely effective.