In spite of RAF pilots advices USAF decided to perform bomb strike on Schweinfurt ball bearing plant without fighters cover.
Both RAF and Luftwaffe has studied this lesson during the beginning of war: bombers without fighters cover can reach targets behind the enemy lines only by the cost of great losses, which at the end of the day will become unacceptable. In 1943 USAF still were thinking they will manage.

14 october of 1943 became the black day for the all strategical aerial offensive to Germany. About 300 single- and twin-motors fighters (Me-109, FW-190 and Me-110) were attacking “Dicker Hund” (it was the nickname for the bombers wave used by the German fighter pilots) all the way to the target and back.
From the 291 fortress took off to the Schweinfurt only 220 has reached the target and drop 478 tons of bombs. Not less then 60 aircrafts didn’t return to their bases and flaming crash sites has created a hundred miles line along the Belgium, Luxembourg, Germany, France. 17 more bombers has landed in absolutely unrecoverable condition.
Total losses were more then 26% from the all forces, participated in that strike. In addition, 121 plane has received less heavy damages.

It was absolutely clear no country can perform constant bombing operations by such a cost. But if RAF decided just to switch time of operation to the night, USAF has chosen other way - they decided to create a fighter able to cover bombers in deep raids behind the enemy line - P-51 Mustang.

Source: Cajus Bekker “The Luftwaffe War Diaries”