The B-25s that were used for the Doolittle raid were modified to allow them to take off of a carrier.
Removal of the lower gun turret 
Installation of de-icers and anti-icers 
Steel blast plates mounted on the fuselage around the upper turret 
Removal of the liaison radio set 
Installation of three additional fuel tanks and support mounts in the bomb bay, crawl way and lower turret area to increase fuel capacity from 646 to 1,141 U.S. gallons (2,445 to 4,319 litres) 
Mock gun barrels installed in the tail cone, and 
Replacement of their Norden bombsight with a makeshift aiming sight. 
This was a one time only deal and shouldn't be used to put a plane in a particular roll (in this case a CV capable medium bomber) that it wasn't used in with the exception of an once in a life time mission.
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