The DC-3 was converted to the Dakota/Skytrain/Skytrooper/C-47 bu adding cargo door, strengthened floor, etc. Later ones were 'converted' back to DC-3, but retained the mods, as Adam says. There was a later super-Skytrain (US designation) that involved larger tail surfaces, engine mods and other changes. Watch "A Bridge Too Far" and look at the fins.