The P-40 crashing in the flight line was an unplanned accident - it was a life-sized mockup powered by a gasoline engine turning the propeller and steered by using the wheel brakes, just like real airplanes, but was specifically designed not to fly. The aircraft shown was loaded with explosives which were to be detonated by radio control at a specific point down the runway. Stunt actors were strategically located and rehearsed in which way to run. However shortly after the plane began taxiing down the runway it did begin to lift off the ground and turn to the left. The left turn would have taken it into a group of other mockups which had also been wired with explosives, but weren't scheduled to be destroyed until later. The explosives in the first P-40 were detonated on the spot in order to keep it from destroying the other planes, so the explosion occurred in a location the stunt men weren't prepared for. When it looks like they were running for their lives, they really were. This special effect was filmed with multiple camera so that it could be reused in other shots in the film, as were all the major special effects.