By late 1945, the B-25 Mitchell outnumbered all other medium bombers in service with the USAAF. Most examples of its Martin B-26 Marauder stablemate had been scrapped immediately after the war was over. Most of the older B-25C, D, and H bombers were immediately released to the Reconstruction Finance Corporation for disposal, but many hundreds of newer B-25Js were retained. Some of these B-25Js remained on active duty, whereas others were placed in storage for possible later use.
Most of the active B-25Js were redesignated as TB-25J, reflecting that they were no longer considered as being combat types. Some were later issued more specialized designations such as ETB-25J, JTB-25J, VB-25J, or CB-25J, reflecting the specific mission assigned to a particular aircraft. For a short period of time, some B-25s were redesignated as AT-24 advanced trainers, but this designation soon reverted to B-25 again. The VB-25 was the designation assigned when the aircraft was used as a staff transport. CB-25 was the designation applied to B-25s used as utility aircraft. Many fighter groups had CB-25s assigned to them for use in a support role. Others were converted as electronics development aircraft under the designation EB-25, ETB-25, JB-25, JTB-25, or NB-25, depending on the level and nature of the modification.
During the immediate post war years, substantial numbers of TB-25s were stripped of their combat equipment and used as advanced multi-engined pilot trainers. They remained in service with the Air Force for many years thereafter, the last example not being struck off the USAF rolls until January of 1959.
TB-25K was the designation given to 117 B-25Js converted as trainers for the operators of the E-1 radar fire control system. The initial contract was awarded to the Hughes Tool Company of Culver City, California late in 1950 for 12 conversions. Later contracts increased the total to 117 aircraft. All military equipment was removed, and a radome was fitted in the front of the transparent nose. The instrumentation for the radar equipment was housed inside a modified bomb bay, and Monitoring equipment for one instructor and the students were installed in the aft fuselage. An astrodome was installed above the navigator's compartment.
Under contract with the Hayes Aircraft Company of Birmingham, Alabama, 75 B-25Js were modified for specialized advanced pilot training under the designation TB-25L. The aircraft were rebuilt from the ground up. All armament and armor was removed, and the pilot's three-piece windshield was replaced by a one-piece windshield that was equipped with wiper blades and an anti-icing system. The front engine hatch was enlarged. Two passenger seats were added forward of the bomb bay, and five seats were installed in the aft fuselage. On some airplanes, exhaust semi-collector rings replaced the "S" stacks on the top seven cylinders of each engine. Deliveries began in April of 1952, and continued through December.
As the TB-25K contract was coming to an end, Hughes was awarded a further contract for the modification of 25 more B-25Js under the designation TB-25M. These were modified from TB-25L aircraft, and were essentially the same as the K model except for the installation of the more advanced E-5 fire control system. Deliveries began in 1952.
From November 1953 through December 1954, Hayes modified an additional 380 B-25Js as TB-25N. They were similar to the preceding TB-25Ls, but were fitted with R-2600-29A engines. Some of these were later modified as VIP transports under the designation VB-25N
Between 1952 and 1954, 979 B-25Js went through the Hayes Aircraft Company for IRAN (Inspect and Repair As Needed). These aircraft were equipped with such features as an automatic pilot, bomb bay fuel tanks, AN/ARN-14 radio gear, dual UHF/VHF, and demand oxygen systems. Sixty planes were fitted with the solid eight-gun nose shell in place of the original transparent nose. All were initially powered by R-2600-29 engines and Holley carburetors, although many were later fitted with Bendix Stromberg Carburetors and the engines redesignated -35.
In the immediate post-war years, substantial numbers of B-25s found their way into units of the Air National Guard. Some TB-25Ks were assigned to certain fighter interceptor squadrons in support of F-89 and F-94 fighters. A few TB-25Ns served with ANG squadrons as weather reconnaissance and personnel transports.