A brief addition/correction: The
B-17 crews did, in fact, have
fighter escorts in daytime. Initially, they were escorted by
P-47 Thunderbolts, which even with drop tanks weren't able to reach all the way into
Germany. However, bomber escort was done in stages so that the bombers were only alone for the outermost leg of their journey; where possible they were met by fresh Allied fighters on their way back to their bases in Britain.
The P-51 was the first Allied fighter able to make the trip from British bases to Berlin and back without refueling, and bomber casualties dropped significantly when it was introduced. The 332nd Fighter Group, the famed Tuskegee Airmen, flew P-51 variants, and those squadrons never lost a single escorted bomber to enemy action - the best squadron record in the war.