Main difference between the H and J was the nose and armament. The B-25J had the full-length nose like the C/D, and was built in both glass-nosed and solid-nosed versions. The 25J didn't have the cannon, but the gunship variants had a TON of .50cal up front (as seen in that pic). The dorsal turret was in roughly the same location (B-25H moved the turret forward just aft the cockpit, whereas the G and earlier it was further aft, as in our C/D).
There was no "I" because it looked too much like a number 1, so was skipped for clarity (same reason that I believe there's never been an "O" variant of an American aircraft). The same is the case with the P-38, which also jumped from H to J.
Incidentally, I could have SWORN I saw a pic somewhere of a 25J that had FOUR .50cal on each cheek (two pods of two on each side) for a total of 14 guns in the nose and cheeks. Will need to see if I can find it.