The original idea was to have the GB for the 'between the wars' period, 1918 to 1939, to allow coverage of the huge variety of aircraft, and rapid developments in aviation during this period. This also allowed a digression from the numerous repeats of the same type of aircraft seen time after time in many of the GBs - '109s, '190s, P-51s, Spits etc.
However, this has now been expanded, for some strange reason, to include WW1, so is aimed at aircraft from 1914 to 1939. The real 'spirit' of the GB is supposed to allow the 'Golden Age' of aviation to be represented, and not, for example, another '109 in a different colour scheme, although this, of course, is likely to happen anyway.