From preparing the bomb to dropping it—the explosion is a few seconds after 8:40. [Video Link]
This silent film shows the final preparation and loading of the "Fat Man" bomb into "Bockscar," the plane which dropped the bomb on Nagasaki. It then shows the Nagasaki explosion from the window of an observation plane. This footage comes from Los Alamos National Laboratory. I have not edited it in any way from what they gave me except to improve the contrast a little — it is basically "raw." I have annotated it with some notes on the bombing and what you can see — feel free to disable it if you don't want it.