The P-61 Black Widow was used to test a number of American ramjet powered missiles and engines. The Navy borrowed two P-61Cs (43-8336 and 43-8347) from the USAAF and used them for air-launches of the experimental Martin PTV-N-2U Gorgon IV ramjet-powered missile, A P-61B (42-39754) was used by NACA's Lewis Flight Propulsion Laboratory in Cleveland, Ohio for tests of airfoil-type ramjets in 1947. The following link, shows some photos of Black Widow test aircrafts:

http://aviacaoemfloripa.blogspot.com.br/2011/02/p-61-black-widow-em-testes-especiais.html


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Interesting. Makes you wonder how much extra speed they got out of the one with the ramjet strapped to its belly!
I once stumbled across photo's of an Baltimore bomber used in similar tests but I can't for the life of me find them now.