There are 50 bridges to fly under (some of them only by chopper) and 15 building gaps to do a knife flight in between (only with planes).

A Buzzard chopper is a good choice for the bridge flights, as this one is pretty small and fast and has the smallest rotor - some tunnels you have to fly through can only be done with this one. The rotor mustn't touch the concrete walls - but you will see if it has worked, as you get an on-screen message with your total count updated.

The Knife Flight can be trained in the flight school - one or two are pretty hard, as the building gaps are pretty small.

Very Important: It is extremely helpful to save your game via the in-game phone while flying your plane to save a massive amount of time in case you destroy your plane and die.
 

Knife Flight Challenges Video Guide

Under the Bridge Challenges Video Guide