Where could I get most information for B-25J insides and outs?
Any good books/walkarounds etc?
Is there also some good aftermarket to be taken count?
Planning to build the Monogram 1/48.

TheMule

Rossagraph have a walkaround of the B25H/J in their catalogue. They are not expensive books and they pack a lot into them.

http://www.jadarhobby.waw.pl/rossa-mitchell-vol1-book-p-10702.html

No need to get it from Poland, many other model/book shops will have it.

AJ Press have several volumes on the B-25. They will have lotts of detail photogaphs and plans but the text will be entirely Polish. Sometimes the photo captions are in English.

Squadron Signal do a walk round on the B 25j Mitchell's and Aires do a detail set for the Acc Min / Italeri B 25b/c, the cockpit deatail can be used on the Monogram kit, Eduard aslo do a PE set for the B 25's.

Hope this helps.

Here are some photos, in the walkaround section, of "Photo Fanny" she's a J :
http://photos.kitmaker.net/showgallery.php?cat=13752&password=

Thanks to everyone on your replies, this will get me forward and I'll post again if something comes up.