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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