The designation F-9 (F for "Foto") was assigned to photographic reconnaissance conversions of existing B-17 bombers. This should not be confused with the F-for-Fighter designation scheme, which was not introduced until 1948.
Sixteen B-17Fs were converted to long range photographic reconnaissance configuration by having their bombing equipment deleted and replaced by photographic equipment. Some cameras were installed in the nose and in the aft fuselage as well. These aircraft were redesignated F-9. Some defensive armament was retained.
The designation F-9A was assigned to an unspecified number of B-17Fs that were converted to photographic configuration in a manner similar to that of the F-9 but differing in some camera details. All of these planes were redesignated F-9B after further camera changes.
The designation F-9B was assigned to 25 photographic Fortresses obtained by adapting some existing F-9As and by converting other B-17Fs.
The designation F-9C was assigned to ten B-17Gs converted for photographic reconnaissance in a manner similar to the F-9, F-9A, and F-9B conversions of the B-17F. I have one photograph of a F-9C in which the chin turret of the B-17G has been replaced by a housing for tri-metrogen cameras. In 1948, the few surviving F-9Cs were redesignated RB-17G, the R standing for "Reconnaissance".
Serials of F-9:
Conversions of B-17F 42-3324, 42-5753, 42-29676, 42-29719, 42-29753, 42-29783, 42-29801, 42-29805, 42-30083, 42-30220, 42-30232, 42-30252, 42-30253, 42-30256, 42-30268, 42-30299.
Serials of F-9B:
Conversions of B-17F 42-2984, 42-6129, 42-6134, 42-6135, 42-6138, 42-6140, 42-6159, 42-6164, 42-6183/6187, 42-6200, 42-6201, 42-29873, 42-29899, 42-29902, 42-29904, 42-29911, 42-29913, 42-29917, 42-29922, 42-30469, 42-30486.
Serials of F-9C:
Conversions of B-17G 43-37689, 43-37711, 43-37914, 43-38155, 43-38162, 43-38168, 43-38649, 43-38651, 43-38653, 44-83626.