An observer on the plane will see the ball curve with the curvature more or less based upon how quickly the plane is turning.

Now lets look at it from someone in a plane flying parallel to the original plane that doesn't turn (and we'll assume the plane with the ball is transparent). An observer on the second plane will see the ball follow almost the identical path as the ball without the turning plane. The path will not be exactly the same because the air is moving with the plane and it will exert a force on the ball in mid-flight, but this force isn't terribly strong--which is why you can throw the ball through the air in the first place.