The horizontal component of velocity of a projectile does not change. The horizontal component of the velocity does not change so the object covers equal horizontal distances in equal times. The vertical component changes due to gravity.
Gravity only acts on the vertical component.
If you are standing off to the side, you will see the object follow a parabolic path. If you are looking down from the airplane, it will look like the object is falling straight down. The object will strike the ground directly below the airplane if there is no air resistance. If there is air resistance, the object will strike the ground a little behind straight below the airplane. The downward component of the motion of a projectile is the same as the motion of vertical free fall.