This book explains it all. The USAAF's strategic bombing campaign in Germany started off based on the Army Air Corps' theory that air power could win the war by itself by destroying key sectors of the enemy's economy and making it impossible for them to continue, and that heavily armed unescorted bombers could defend themselves and do the job without prohibitive losses. It slowly dawned on some of the air generals that that wasn't really working, but what was working was that the bombing raids were drawing the Luftwaffe fighters into combat, where escort fighters could engage them and impose losses. The allies needed to deeply attrit the Luftwaffe in order to succeed in the D-Day landings, and the Luftwaffe just did not come up to meet fighter sweeps over the Continent, they only engaged to defend German factories. The bombing campaign's original goal of destroying German industry became secondary to the mission of drawing the Luftwaffe into air battles where it was greatly outnumbered and slowly depleted to the point where it could not interfere with the D-Day landings.
In the Fall of 1943, the Combined Bomber Offensive was failing. German aircraft production was rising, not falling. Many more Allied planes were being shot down than German. Deep penetration raids by unescorted bombers to reach German factories suffered intolerable 20% losses. Nothing helped: not more guns on the bombers, not different flying formations, not larger raids. The one thing that helped was the arrival of long-range escort fighters (chiefly P-51 Mustangs equipped with drop tanks) that were able to accompany the bombers to the target. The real turning point came in early 1944, when it sank home that attritting German fighters was the important thing and escorting fighters were freed to pursue enemy fighters away from the bombers. Now, if a German fighter dived away from the bomber formation, the escort fighter would follow it right down to the treetops to ensure its demise.
It's ironic that air combat is quintessentially about maneuver at the tactical level, but at the operational and strategic levels the bombing campaign was a grinding war of attrition. It was the trench warfare of WWII--yearslong combat week after week, constant casualties, and little visible change in the military situation.
One interesting tidbit: The German pilot training program was relatively modest in 1939 because they anticipated a short war. They never caught up. Towards the end of the war the German squadrons generally had enough planes, despite the Allied attempt to destroy the German aircraft industry. By heroic effort, the industry had been repaired and dispersed and hidden, and production kept increasing until 1945. But experienced pilots and new pilots with sufficient training became hard to find. German training in bad-weather flying was particularly weak, leading to many non-combat losses. German pilots went into combat with about half the hours in the air that American pilots did.