Shortly after the end of the war, large numbers of surplus Mitchells were made available for sale through the Surplus Property Division of the Reconstruction Finance Corporation, a Depression-era government agency. The RFC was supplanted in 1946 by the War Assets Administration (WAA).
Initially, the USAAF released older B-25Cs, Ds, and Hs for disposal, retaining the newer B-25Js in active service or in storage for later use. Also, the Navy released most of its PBJ-1 aircraft, most of which were B-25Js. The Air Force did not release its inventory of B-25Js for disposal until the late 1950s.
Once sold on the commercial market, many of them were converted to executive transports for use by private corporations, with all their military equipment being removed, the bomb bay being faired over, and plush interior accommodations with soundproofing and airline-type seating being added. Some even had wingtip tanks. Very often, the noisy individual cylinder "S"-type exhaust stacks were replaced by one or two semi-collector rings exhausting to ports on the outboard side of the engine nacelles.
In 1949, North American Aviation thought that might be a real market for converted Mitchell executive transports, and undertook a program to modify standard B-25s into transports. They obtained a surplus B-25J (44-30975) and modified it as a prototype executive transport. The forward fuselage was redesigned to make it wider and nearly four feet longer, making room for four more seats in the cockpit section. The bomb bay section was reworked to carry a bunk and cargo storage. Four additional seats were added in the waist section. It was assigned the civilian registration of N5126N, and flew for the first time in February of 1950. Unfortunately, during a cross-country tour, the aircraft crashed on March 1, 1950, killing all seven people aboard. This crash, and the onset of the Korean War a couple of months later, led to North American Aviation abandoning its efforts to make a commercial transport out of its B-25.
A considerable number of surplus Mitchells found employment with contractors to the US Forest Service as fire-fighting aerial tankers in which fire-retardant chemical tanks were installed in the bomb bay. However, after four crashes during the 1960 fire season in the western USA, the Mitchell was declared unsuitable by the US Forest Service for use as an aerial tanker, and most were withdrawn from use. The Mitchell had better luck in Canada as an aerial tanker, G7M Aircraft of St. Albert, Alberta using them until 1992.
Surplus Mitchells also were briefly used as aerial agricultural pesticide sprayers. All military equipment was removed and underwing or undertail booms were fitted for the dispensing of pesticide. The pesticide tanks were installed in the fuselage and wings as well as in the bomb bay. However, environmental concerns about the widespread use of pesticides led to the rapid decline in the demand for agricultural B-25s, and most examples were converted to other uses or sold.
A considerable number of Mitchells ended up being employed by the movie industry. B-25H serial number 43-4643 (civil registry N1203) was used by Tallmantz Aviation for film work. It was fitted with a new, transparent nose for use in Cinerama film work. Substantial numbers of Mitchells appeared in Mike Nichols's film Catch 22, a movie adaptation of Joseph Heller's black comedy about wartime operations in the Mediterranean. Other Mitchells were used as camera ships for such movies as Battle of Britain and 633 Squadron.
There is a listing of all B-25 Mitchells which entered postwar civilian service available at the Aero Vintage Books website. It is based largely on Scott Thompson's book B-25 Mitchell in Civil Service. For a more complete story of each of these planes, check out the book. I recommend it highly.
As many as 150 Mitchells still survive today, either on display in museums or as flying museums operating with such outfits as the Confederate Air Force. At last count, about 40 Mitchells are still airworthy. There is a B-25 Locator at the Aero Vintage Book website. It lists the location of all currently-surviving B-25s, including those in museums as well as those which are still airworthy.