Set up waypoints & loadout for the bombers
In the example below, I've set a very short route for this tutorial. Somewhre on the route, place a waypoint directly on the target. In this case it's an airfield. Make that waypoint a GATTACK.
Something to be careful of: If you set the waypoint prior to the GATTACK too close to the GATTACk waypoint, the bombers may not release their bombs.
Why? Because there's not enough distance for the bombs to hit! The first level bombing mission I made was from 10,000m (about 30,000ft) and I couldn't understand why it wasn't working. The bombs travel quite a long way horizontally, so give them plenty of distance:
Level bombing - almost no complications
Some things are a bit tricky to get right in the FMB, but level bombing comes with a couple of potential catches - waypoint length and remembering to give the bombers a loadout!