This is the B-17F “Spotted Cow” Flying Fortress assembly ship of the 384th Bomber Group based at RAF Grafton Underwood in Northamptonshire, England. The assembly ship's mission was to serve as the reference point for hundreds of bombers taking off for a mission to fall into groups for the long flight to the target. As soon as the mass formation would take shape, the assembly ship returned to base. The “Spotted Cow” flew 61 combat missions.
Taken from the excellent 'Fans of the B17' Facebook page
Anyone know of any other pics of Assembly Ships?