https://www.strategypage.com/htmw/htairfo/20090823.aspx
August 23, 2009: For the third time in the last decade, the U.S. Air Force is looking at using commercial aircraft as bombers...
Militarizing the 747 means more than a new paint job and the addition of air force radios, radars and other electronics. The big changes are internal, where the fuselage has to be beefed up to handle the unique weight and shape of bombs and missiles. Freighter versions of the 747 (which carry about half the world's air freight) use containers, for the most part, because it is more efficient.
Another problem that needs some internal revisions is the fact that commercial transports tend to be low wing (with the fuselage above the wings) while bombers tend to be high wing (fuselage under the wings). This is done so the bomb bay will be close to the center of gravity (which tends to be where the wings meet the fuselage. Thus, when the bombs are dropped, the center of gravity is not changed. You need a stable center of gravity to fly the aircraft. When a freighter is loaded, you distribute the containers (adjusting for the weight of each), so that the center of gravity is not distorted too much (or more than the flight control system can handle.) Thus the militarized 747 would need a system for launching the bombs and missiles in such a way that the aircraft does not become impossible to control.
The commercial version of the Boeing 747-8F cost about $280 million each. A militarized version would probably cost closer to half a billion dollars each