I didn't find any "F's" here. http://www.armyairforces.com/tails-29.asp

Joanne Smedley wrote:

Hello, I am hoping someone might be able to identify the squadron an
aircraft serial number 22-4621 and the letter F painted above it which
appears on a photo I have come across. Two B29s staged at Tarakan,
Borneo, on 14 August 1945. No other information available.

Regards, Joanne Smedley