By the end of World War II, the B-25 Mitchell was still an extremely useful platform, but with higher-performing A-26s in service and B-42s on the horizon, large numbers of B-25s were headed for storage and disposal. When new hostilities in Europe suddenly heated up, the decision was made to bring the B-25s out of storage and reverse the process. To make them more effective on the battlefield, North American designed a four-jet engine installation (using Allison J-35s), replacing the Mitchell's two R-2600 radial engines. Few other changes were made in order to keep the modification and testing program as quick as possible. The first B-25R flew in June, 1946, and B-25Rs began to enter squadron service in November of that year.
I got the 1/72 Airfix Mitchell as a prize from a previous GB (thanks, Bill!). Since I believe in building what I have on hand, I chose that as my starting point. The Smer Me 262 kits were the cheapest way I could get four jet nacelles. I am slowly working my way through the design of the engine mount. I chose to mount the engines in such a way as to preserve the original main landing gear installation, and they needed to project well forward to maintain CG. I still have a long way to go to make it look reasonable.