There were a number of symbols, numbers, and letters painted on the 95th BG B-17's to help identify the aircraft to the Allied forces. 

                                             Photo:  Aircraft 42-31063 "Spook Six" - 335th BS

U.S. Aircraft Symbol

This consisted of a white star within a blue circle with white horizontal tabs (see example above).   This identified the aircraft as a U.S. plane.  This was painted on the sides of the aircraft, as well as on the top and bottom of the left wing.

Bomb Group Symbol 

B-17's of the 95th Bomb Group were painted with a "B" in a square on the tail of the plane and on the wings.  This was either a white B on a black square or a black B on a white square.  This varied depending upon the plane's color.  Seen below are examples of both.

Aircraft Number 

On the tail of the B-17 aircraft was the aircraft number.  This was a unique number that identified each B-17. 
The first number of the aircraft number identified the year that the plane was manufactured,
i.e. 2=1942, 3=1943,  and 4=1944.

Radio Call Letter  

The Radio Call Letters were painted on the side and tail of each B-17 so that each plane
within a squadron could identify and communicate with other planes during missions. 
Each plane contained a single alphabetic Radio Call Letter.  In some cases planes were
reassigned and given different Radio Call Letters.  For example, aircraft 42-3317 was with
the 412th BS and was painted with "Y" and "V" radio call letters at different times.

In the above picture, aircraft 42-31063 "Spook Six" (335th Bomb Squadron), had a Radio Call Letter of "T". 

Squadron Code Letters and Aircraft Noseband Colors

For some early period during the war, B-17s were painted with Squadron Code letters.  In addition, they experimented with painting the noseband of the plane to identify the Squadron as well.

Squadron Code letters and colors of nosebands of 95th BG Squadrons

     Squadron   Code      Noseband color
 
        334th        BG            yellow
        335th        OE            dark blue
        336th        ET             light green
        412th        QW           red

              Squadron Code Letter "OE" on the 335th Aircraft 44-8040

NOTE:   The call letters and squadron codes were only painted on the planes in the beginning of the war. 
Later, there wasn't enough time to get all this painting done with the number of planes being lost and replaced.
It was also determined that the extra paint reduced air speed and used precious fuel.