No H came direct from the factory.
The G model was a field mod.
+1 on the H as a factory mod - they also moved the dorsal turret just behind the flight deck, added two 50 more cals, lighter 75mm and gave the navigator single controls at his station. And no co-pilot...

The B-25G (only 400 25C's were converted) was basically a 25C, with more armour plating, more fuel capacity, and was slightly heavier. Mods were done at operational locations instead of retiring the C outright.