After 16 years of development and an investment of somewhere around five billion dollars, the Air Force has decided to retire their Airborne Laser test bed, a 747 fitted with a rotating turret in the nose that could shoot down ballistic missiles from long range with a giant chemical laser.
The Airborne Laser test bed (ABL) was an ex-Air India 747-400 freighter that Boeing purchased and refitted with an optical turret in the nose, backed with a megawatt-class chemical oxygen iodine laser. While in flight, the ABL would use an infrared tracking system to detect the launch of ballistic missiles, target them with a secondary kilowatt-class laser that mapped out disturbances in the air between the plane and the missile, and then fire up its main laser system and blow the missile to pieces with a 3-5 second burst of energy.
The ABL sounded great in principle, but the problem was that it didn't really work. Or at least, it didn't work well. At relatively close range (within a few hundred miles), the system was able to successfully destroy ballistic missiles during their initial boost phase whenever it wasn't having electrical or mechanical issues. However, a few hundred miles is closer than the ABL was ever likely to be to a ballistic missile launch site, and in order to get more distance out of the laser, it would be necessary to boost its power output by a factor of at least 20. As it was, the giant vats of toxic chemical that powered the laser were only able to deliver a couple dozen full-power shots at best, and the ABL would then have to land to take on more laser fuel.
As cool as it was, the ABL ultimately just didn't make enough sense — either financially or from a combat effectiveness standpoint — to try and field in an operational capacity. This is not to say that high-power laser research isn't ongoing, but the ABL itself will be placed in long-term storage at the "Boneyard" in Arizona to wait for the day when mounting a laser cannon on a 747 does make sense.