B-29, P-51 and more, help with photos

[photo-war]

Dear forum members! Looking for help with these photos, all are originated from the same photo album. I guess they were taken post war somewhere in USA, may be they could be dated as well.

#1 and #2 - what models/types are these planes? Post war planes ain't well known to me ; (

#3 B-29 "Superfortress" - any chance to tell from what unit is this plane? Note that there is a # on a tail under big "3", this is 521670 or 521870.

#4. What variant of P-51 is this?

Thanks in advance for any comments on these images!

Best regards,
Alexander