The 747 is one big target. How about a SR-71?
Nope, the reason for a 747 (or C-130, where we already have lasers mounted, I've heard), is that to get the output needed for a laser weapon,
you need a huge heavy energy source, and a long lasing chamber.
You mount the laser down the entire length of the airplane, which has to be
big to lift the energy source, and aim it by aiming the plane.
A small, light energy source (even small and light enough to put a laser weapon on an SR-71) is simply not available.