Click on frame 50 and press F6 key. Then, place the "military aircraft" on the right position. See the picture below.
Right-click anywhere on the gray area between the two keyframes on the timeline and choose Create Motion Tween from the menu that appears.
Create a few "military aircraft" in an equivalent way like we have created the first "military aircraft".
Now it's time for thunder. Create a new layer and name it thunder. Then, using the drag and drop technique movie it below the clouds layer. See the picture below.
Take the Rectangle Tool (R). In the Colors portion of the Tool panel, block the Stroke color by clicking on the little pencil icon and then on the small square with the red diagonal line. For Fill color choose white color and draw a "rectangle" about 300x300px. See the picture below.
While the recatngle (that will represent our thunder) is still selected, press F8 key (Convert to Symbol) to convert this rectangle into a Movie Clip Symbol.
Double-click on the movie clip on stage with the Selection tool (V). You should now be inside the movie clip.
While the recatngle is still selected, press again F8 key (Convert to Symbol) to convert it again into a Movie Clip Symbol.
Using the mouse and drag and drop technique, move frame 1 on frame 20.
Take the Selection Tool (V), click once on the Movie Clip (recatngle) to select it and go to the Properties Panel (Ctrl+F3) below the stage. On the right, you will see the Color menu. Select Alpha in it and put it down to 0%.
Then, press F6 key eleven times. See the picture below.
Click on every second frame, take the Selection Tool again (V), click once on the Movie Clip and go to the Properties Panel below the stage. On the right, you will see the Color menu. Select Alpha in it and put it down to 76%. See the picture below.
Click on frame 70 and press F5 key.
Go back on the main scene (Scene 1) and test your Movie (Ctrl+Enter).
Have a nice day.
Download source file (.fla)