NOt quite.

The bullet would not 'instantaneously' slow down. It has a given amount
of energy and is capable of supersonic speeds. The question would be
how long would it take to slow down to the Concorde's speed. If you
guess a second, and the bullet was fired along the flight path of the
airplane, it would have fallen 16 feet be the time it was traveling the
same speed, and another number of feet before it slowed down enough for
the airplane to catch up to it.

During WW2 some airplanes were said to have hit themselves with their
own bullets, but I think you can work out how that might happen with an
airplane diving under its own line of fire.

Finally, if you think you knew the answer, why did you post the
question?