With bullet/plane(s) relative horizontal speed of 820ft/s, the other
plane reaches the bullet position in (50x3)/820 = 0.183 sec (pretty
slow bullet). In that time the bullet falls a vertical distance of 0.5
x 32 x 0.183 x 0.183 ft = 0.536 ft. If the messer plane bottom was at
least 0.537 ft (about 7 inches) below bullet firing vertical position
its gona hit the other plane.

Ignoring air friction, whether the planes are moving or parked on the
ground with same separation it does not matter. It is only the relative
velocity of the bullet to the planes that counts. But with backward
airstream and downward friction the bullet will fall slower down than
in vacuum - so better chance of hitting the plane behind.

Is this your night school physics home assignment and you are cheating
here?