I recieved a response from the gentleman I emailed about this picture. Here is what he had to say:
Believe that the tail number has six digits, perhaps it is 210511 but
I am not sure of the 11. This would make it 42-10511, which is a
P-40L.
Not sure why the American flag is on the side of the aircraft, but it
might be there for recognition purposes. The white star might have
been hard to see under certain situtations. Many of the P-40Ls were
exported under Lend-Lease and this might have been a publicity photo.
The plane in your picture does have RAF-style camouflage. I have a
picture of a P-40F with USAAF insignia but with an American flag
painted on it. The caption says that the plane had originally been
sent to the RAF under Lend-Lease but had been returned to the USAAF in
North Africa. Perhaps your plane is also in this category.
Before Pearl Harbor. many US aircraft had American flags painted on
them to indicate our neutral status, but this photo looks like it was
taken after US entry into the war, perhaps sometime during 1942, at
the time after the original red center had been removed from the white
star but before the addition of the white rectangles.
I hope this helps!