The -J was the last B-25 production model manufactured by North American Aviation (NAA). Almost 4,400 B-25Js were built, far more than any other model. The B-25J was an improved B-25H but looked like a cross between a B-25C and a B-25H. The solid nose was replaced by a 'greenhouse' with one fixed and one flexible .50-cal. machine gun. The bombardier was returned to the crew bringing the total to six men aboard.
NAA modified 800 -J models by installing a solid nose in place of the bombardier's 'greenhouse.' The solid nose housed eight .50-cal. machine guns bringing the total to eighteen--fourteen of which were forward firing for strafing missions. NAA also produced field conversion kits for the solid nose version. The Royal Air Force received 316 B-25Js as "Mitchell III" and converted most (or all) of them to the solid nose version.
The B-25J; powered by improved Wright R-2600-29 radial engines, first flew on 14 December 1943. Production of the aircraft was continued until August 1945 when 72 incomplete aircraft remaining on contract were canceled. These aircraft were flyable but not counted in the Army inventory. 4,390 B-25Js were built with 4,318 of them being delivered to the Army or diverted/direct delivered to Allied nations.