For the third time in the last decade, the U.S. Air Force is looking at using commercial aircraft as bombers. This time around, it's mainly a matter of cost, with the next generation heavy bomber likely to cost over a billion dollars each, and only carry 30 tons of bombs or missiles. The idea of militarizing 747s first started gaining traction three decades ago, as cruise missiles showed up and many air force analysts did the math and realized that it would be a lot cheaper to launch these missiles from a militarized Boeing 747. The freighter version of the latest 747model, the 747-8F, can carry 140 tons of cargo. After militarizing the aircraft, you would still be able to carry about a hundred tons of missiles and bombs.
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. But with the ground mapping radar, targeting pod and in-flight refueling, it could stay in the air for over a day (with a relief crew, and crew rest quarters in the upper deck) and provide the kind of persistence, surveillance and fire power the troops appreciate most from the air force.
One major development that has made the militarized 747 attractive is the smart bomb, which has turned just about every warplane into some kind of bomber. There are no more pure fighters, most warplanes can now carry bombs. And most of the bombs are of the "smart" (GPS or laser guided) variety, meaning that not much special training or equipment is needed for an aircraft to be a "bomber." Since this trend began in the 1970s (laser guided bombs) and 1980s (ALCMs, Air Launched Cruise Missiles), it has often been suggested that cheaper civilian aircraft be used to carry these more intelligent weapons.
A specially equipped B-747 would be cheap, you could buy ten or more for the cost of a B-2. And the B-747 would carry more bombs just as far as the B-2. The B-747 would not be stealthy, but neither is the AWACS air control aircraft that flies a hundred kilometers or so from enemy territory to direct the air battle, nor the KC-135 and JSTARS aircraft that fly over the battlefield. Both of these aircraft, and many others long used for combat zone missions, are basically militarized civilian transports (the Boeing 707, although the KC-135 actually came first, and then morphed into the 707).
If you want to send bombers right into the teeth of the enemy defenses, use smaller warplanes you already have. For heavy lifting, you could use the 747 bombers. But this sort of solution is not popular in the air force. It's not that the air force is not interested in getting the most bang for their buck. But the air force is run by pilots who have all personally experienced the dangers of flying high performance aircraft into harm's way. That kind of experience tends to make quite an impression on people and they are reluctant to give it up to a bunch of flying trucks carrying robot bombs.
Meanwhile, the USAF has also changed its tactical doctrine in the last decade, and is no longer willing to go in low after ground targets. In the 1999 Kosovo campaign, most of the bombing was done from three miles up. You can only use smart bombs and missiles at this altitude. Bombing from that far up only protects you from anti-aircraft artillery, not missiles. ECM (Electronic Counter Measures) and agile flying are required to defeat SAMs (Surface to Air Missiles.) A 747 cannot outmaneuver a SAM like a F-15 can. But we already have the equivalent of 747s doing a lot of the bombing from a safe distance; the 1950s era B-52s.
Smart bombs and missiles are more expensive than plain old bombs. What makes a smart bomb smart is an add on kit that provides the guidance, and sometimes a small rocket to give it a little more range. These kits cost anywhere from $10,000 to over a $100,000. Missiles cost from about $50,000 to a million or so dollars each. A bomb, all by itself, costs about a dollar a pound. But the original idea behind smart bombs was to save planes and pilots from ground fire by requiring fewer bombs to be sent towards a target in order to destroy it. Over the last few decades this was accomplished, with the air force realistically seeking the formerly unattainable goal of "one target, one bomb."
Meanwhile, another unexpected side development was the fact that GPS guided bombs made it possible to deliver accurate, safe (for the friendly troops) bombing missions in any weather. The laser guided bombs were foiled by smoke, fog or mist. But GPS bombs came through in any weather, and could be dropped from a high (out of ground fire) range. If you were over a battlefield where the enemy did not have high-altitude missiles (shoulder fired missiles are low altitude), any aircraft could successfully drop the GPS guided bombs. In the past, it took up to 300 bombs to destroy a target from that altitude (and a dozen or more even if you came in real low).
Using commercial aircraft as bombers is nothing new, and has been going on since the invention of aircraft. All an air force does is design an aircraft more suited to dropping bombs. But in many parts of the world, like Sudan, commercial transports are still used to deliver bombs.