One thing often forgotten is, that Krupp (who built the guns) did not wait until the Kriegsmarine officially thought of or ordered a gun. The design was independent and Krupp indeed constructed the 38 cm gun in 1934 not exactly knowing if the KM would ever buy this design.
Before WW1 he offered designs which never come into service like 15-cm-L/60 guns or a 10,5-cm-MACHINEGUN able to fire 10 rounds with a 1sec. per shot ROF...
Unlike the Groundforces or the Luftwaffe the KM did not really initiate new gun designs. They asked Krupp what he could deliver...