These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
While it is shown that at the time, the USAAC lacked a fighter that could fly with the B-17s all the way to their targets and back, in 1943, when the film is set, it was P-47 Thunderbolts providing escort for the B-17s, not P-51s that are seen. While the film crew may have been gambling that no one would notice since the P-51 is one of the most iconic aircraft of WW2, there were P47s in flying condition at the time of filming, so there wasn't much of an excuse.
A justified version comes into play with the Bf-109s seen in the film, which are in reality Ha-1112s, a Spanish version of the Bf-109. At the time of the filming, no Bf-109s in flying condition even existed, let alone being available for a film crew to use.
Also played with with the bombers themselves. The B-17s in the film are all B-17Gs, mocked up to look like B-17Fs (which the Memphis Belle really was). The film crew did a convincing job of it too, but they still forgot to get rid of the bubble on the nose (in front of the windscreen, which was used for night navigation so the navigator could see the stars), which marks all the bombers as very late model B-17Fs.
A crippled bomber crash lands and explodes on the tarmac in the opening sequence. The crew can be seen trying to escape the bomber just before the explosion.
The lead bomber, Windy City, catches fire and explodes directly in front of the Memphis Belle. After they fly through the debris, entrails are found greased on the windows of the front of the bomber.
The deputy-lead bomber, C-Cup, falls out of formation when its nose is blasted off. Its bombardier is seen falling out of the front of the bomber - his parachute not yet attached.
A crew on their first mission, Mother & Country, is ripped in half by a crashing fighter. Danny, who had befriended his counterpart on the rookie crew, channels his radio to be sure his friend is alive. Instead, he listens as the rookie crewmen scream while falling to their deaths.