So the conventional wisdom goes. However I don't think they went into their bombing campaign completely blind. They did know how the German bombing offensive went and why they switched to night bombing, and what happened to English daylight bombers and why the British had switched to night bombing, and then explicitly to area bombing in early (February) 1942, followed by the "Dehousing" policy memo in March of that same year. Some time between them and the first US fighter escort of a B-17s mission later in 1942, it had begun to dawn on some of them that there would be a need.
The American Army Air Force generals, and the bomber mafia specifically, still believed in their doctrines like "the bomber will always get through" and their perceived super weapons especially the B-17 and the Norden Bombsight, and they still believed in precision daylight bombing. But a nagging counter narrative was already taking shape even in 1941. The British were doing their best to talk the Yanks down from the ledge (as they saw it) and give up the idea of bombing in the daytime altogether. By mid 1942 this was becoming a rather heated argument. In theory B-17s were so extraordinary they could get the job done, but combat experience all through 1942 did not indicate any aircraft that could attack German targets alone in the daytime, except maybe Mosquitos.
The middle position between night bombing and unescorted daylight bombing is
escorted daylight bombing. So they were hedging their bets a bit in supporting the development of fighters which looked like they could perform escort duties, in spite of the long and painful development cycle of both planes (P-38 and P-47). P-38s incidentally were being used to escort B-24s and B-17 in the Med from mid November 1942 and all through 1943. The first escort mission by P-38s was on 19 November 1942, they escorted a group of B-17s bombers on a raid over Tunis. I think it was already quite clear in early 1943 that B-24s in particular needed escorts over Tunisia and later Sicily and Italy etc. They were surprisingly effective at wiping out air bases - I think more German and Italian planes were destroyed on the ground by the heavy bombers in winter 1942/43 than by any other single method in that Theater. But they needed help to survive, they were taking losses even with escorts.
The proof that escorts were required over Northern Europe may not have been
irrefutable until after Schweinfurt etc., but remember at the time of Schweinfurt many of the raids were already being escorted, it's just that the best targets were
beyond the range of the escorts, tempting them to send the bombers out alone in the last leg of their trip. The Regensburg raid was escorted by 87 P-47s from the 353rd and 56th Fighter Groups as far as Belgium. The Schweinfurt raid had 88 P-47s and 96 Spitfires watching their backs on the way in,. They were then met in "withdrawal support" by different fighter groups (totallying 93 P-47s and 95 Spitfires for Schweinfurt) on the way back out again. The P-47s of the 56th Fighter Group were escorting bombers (so called "
Ramrod" missions)
from 29 April 1943. 78th Fighter Group were flying escort missions from Duxford also from April 1943.
So it's not like the notion of escorting B-17s and B-24s was new in 1944. That is just a shorthand version of a more nuanced reality. Given that they were flying heavy bomber escort missions with P-38s in the Med from November 42 and with P-47s in April 43, I think they conceived of the idea of using them in that way maybe a little earlier than that.