You raw statistics are great. The problem is that raw statistics do not show the real story.
What happened did not happen because of the change in aircraft. It happened because of a change in leadership and the attending change in tactics. And no, the change in aircraft is not what allowed the change in tactics.
The 8th AF could have been providing long range escort of deep penetration bombing raids from the beginning. They specifically chose not to. The 8th AF was good at compiling statistics. They turned thousands of young men into statistics, needlessly, between September of 1942 and February of 1944.
First, they sent their long range fighter to North Africa, and the Mediterranean. They kept the short legged P-47 in Europe, and moved the P-38. They were convinced that heavy bombers did not require escorts. Second, they remained convinced until late 1943. The 8th AF leadership was incompetent at best, myopic and negligent is probably a far more apt description.
No real effort was made to equip the P-47 with enough fuel capacity to provide long range escort until it was almost too late. Further, the 8th AF did not request a significant number of P-38's, either.
As an aside, if the 8th AF leadership had been competent enough, and influential enough, they probably could have made enough demand for long range fighters that the P-38 would have been properly second sourced. Instead, they only managed to use up B-17's fast enough that Lockheed was using 50% of the Burbank plant to produce B-17's for the 8th AF to get trashed due to their grossly mismanaged efforts. Double the number of long range escorts would have cut the bomber losses enough that the B-17's coming from Burbank would not have been needed.
How completely shortsighted was the 8th AF? Well, you mentioned the 20th and 55th Fighter Groups. Those two groups went operational in the weeks after "Black Thursday". Despite their inexperience, both in the air and on the ground, which resulted in 20% of their strength never leaving the ground, and then as much as 40% of those that did being early returns, they cut bomber losses to fighters dramatically. Often with as few as a dozen P-38's making the whole trip. Those two groups went operational in 1/3 the time normally allotted, in October and November of 1943. There should have been, and could have, been a dozen such fighter groups operational as early as the first quarter of 1943, considering that the P-47 could have been fitted with drop tanks, if anyone had been so intelligent as to request it with any authority.
What happened in late 1943 and early 1944 is that the leadership changed, and the tactics, even the reason for the very existence of the 8th AF changed. The 8th AF went from an air force trying to bomb German assets on the ground, to an air force dedicated 100% to taking control of the sky over Europe. The bombers were merely bait, a reason for fighters to be all over the skies. Sure, they dropped a lot of bombs, about 1/2 of which, or more, missed their targets. But they drew enemy fighters, which, forced to engage the bombers, became vulnerable to fighters.
It wasn't the P-51 itself, which by the way suffered massive teething issues (stick reversal at low speed, fouled plugs, cracked cylinder heads, etc) that were covered by the existence of other fighters in decent numbers. It was the fact that guys like Doolittle decided they were going to cleanse the sky of German aircraft. You could have hung drop tanks on any competent fighter aircraft in decent numbers with decent pilots in the cockpit in place of the "famous" P-51 and done exactly the same thing.
The P-51 was given every advantage. They did only segmented escort, they weren't, for the most part, tasked with escorting bombers for an entire mission. They were sent directly to a rally point, where they were handed bombers to loosely escort for certain legs of a mission. The P-51 did not handle the entire missions, nor were they tied to close escort on most of their missions. The replacement P-51 pilots were sent to "clobber college", the forerunner of the "Top Gun" type schools.
It is always amusing to see the P-51 attributed such magical powers as "the first long range escort fighter" and "the fighter that beat the Luftwaffe". It was a complete change of leadership, and the attending change in tactics, that made it possible to completely defeat Germany in the air. Had the P-51 been introduced, and then tied to the same doctrine and tactics that the P-47 and P-38 had been tied to, "Operation Big Week", and more importantly, "Operation Overlord" would never have been possible. The war in Europe might have dragged on another year or more.
