Well, the 14 IXB's built were all constructed during those years and assigned to 2nd Flotilla in Wilhelmshaven. However, other than
U-64 (entered training on 16th Dec, '39) they all entered training and frontboat status during 1940.
However, according to the copy of Flotilla.cfg from my install of NYGM 2.2, it shows the IXB should only be available from Oct '40 onwards?