Civilian use

Surviving aircraft were offered to civilian governmental agencies, or declared surplus and offered for sale on the commercial market.

An RF-61C (ex-F-15A, serial number 45-59300) was used by NACA at Moffett Field in California to test some early swept-wing designs by dropping recoverable aerodynamic test bodies from high altitude. This program was later joined by F-61C serial number 43-8330, borrowed from the Smithsonian Institution for the duration of the tests. These drops were carried out over Edwards Air Force Base in the Mojave Desert in California. F-61B-15NO serial number 42-39754 was used by NACA's Lewis Flight Propulsion Laboratory in Cleveland, Ohio for tests of airfoil-type ramjets. F-61C-1NO 43-8357 was used at Ames as a source for spare parts for other F/RF-61 aircraft.

A few other Black Widows also ended up in the civilian market. P-61B-1NO serial number 42-39419 had been bailed to Northrop during most of its military career, who then bought the aircraft from the government at the end of the war. Having the civilian registration number NX30020 assigned to it, it was used as an executive transport, as a flight-test chase plane, and for tests with advanced navigational equipment. Later it was purchased by the Jack Ammann Photogrammetric Engineers, a photo-mapping company based in Texas; then in 1963, it was sold to an aerial tanker company and used for fighting forest fires. However, it crashed while fighting a fire on 23 August 1963, killing its pilot.

The last flying examples of the P-61 line were an F-15A Reporter (RF-61C) 45-59300 and the "spare parts" F-61C 43-8357. The latter was rebuilt as a high-altitude mapping airplane, assigned the civilian number N5094V, and offered on the commercial market; however, it attracted no buyers and was finally scrapped in 1957. The RF-61C, originally given the civilian registration N5093V, was sold to Compania Mexicana Aerofoto S. A. of Mexico and assigned the Mexican registration XB-FUJ. It was then bought by Aero Enterprises Inc. of California and returned to the USA in 1964 carrying the civilian registration number N9768Z. The fuselage tank and turbosupercharger intercoolers were removed; and fitted with a 1,600-gallon chemical tank for fire-fighting it was purchased by Cal-Nat at the end of 1964, and in turn in March 1968 by TBM, Inc., an aerial firefighting company located in California (the name of the company standing for the TBM Avenger, the company's primary equipment). It was destroyed in a takeoff accident on 16 September 1968.