I'm not sure of the 18 December 1939 mission profile for the Wilhelmshaven raid, but Bomber Command had a bad habit of not massing aircraft in the early years of the war. Aircraft allocated for a certain mission were given very general target attack instructions, were allowed to plot individual courses and altitudes, taking off and attacking individually to comply with certain timeframes, but there was no massed time-on-target (or time over the target area when multiple targets in the same vicinity were to be engaged). German ADA was often able to pick off attacking aircraft one-by-one, and were fully alert by the time the last few meandered over the target.
