How do you get the IXB ship (1939)?

When I look at the timeline it shows that in 1940 there are 5 different subs available. The 2A, 2D, 7B, 7C and 9B. I seem to have access to everything but the 9B. How do I get it? Ive run 7 patrols so far and just now can purchase the 7C. Do I have to save up money for the 9B to show up on the menu? I don't want to spend it on the 7C if I can jus wait another patrol or two and get the bigger and better sub. Anybody have any info? Im part of the first fotilla by the way. Do I have to switch to the 7th to get it? I know the 7th starts off with a 7B sub right from the start because many of their missions is in the atlantic.