It is a duplicate, but don't be sorry. The B-25 is one of the more-missed aircraft in the game for a lot of folks. And welcome to AH and to the boards!
As to why it isn't here, best answer I can come up with is this. . . As with any game, AH had to begin somewhere. It started with representative samples, and has built upon them and filled in gaps as time goes on.
So, right now there is no B-25. There is a B-26, which is a "representative sample" of the "two-engined American medium bomber." There is an A-20, which is a "representative sample" of the "two-engined American gunship (attack-bomber)."
With both the American twin-engined attack and twin-engined bombers "covered", not to mention the German Ju-88, Japanese Ki-67 and British Bostons (the last being a variant of the A-20), the B-25 has simply been low on the priority list.
I am sure HiTech will get to it eventually. Just know that fighters receive much higher priority, and there are a few other gaps in the plane set that are IMO more significant that will probably be filled first. So the wait will likely be a (very) long one.