There are three full size flying mosquitos used in the film, all converted from target towing versions, with 3 more static non flying versions one of which was destroyed in a crash sequence.
For the bombing run shots miniatures were used supervised by Tom Howard. IMDB claims the miniatures were 1/48 scale however I believe this is erroneous as they would have been much larger than that.
There are two miniature sequences in the film, one depicts the practice bomb runs in Scotland where there is a big white cross painted on the rock cliff and the actual mission in a Norwegian fiord both represented by a large miniature environment. The fiord also features a power plant also in miniature over which hangs a towering rock face which eventually collapses down on it destroying the installation. While the rock collapse is quite well done, the paint gets chipped off the rocks as they fall and collide revealing the white plaster underneath from which they are made.
In the final mission sequence there is a great deal of anti aircraft pyrotechnics, which in some shots appear to be hanging from wires. In fact while the action of the miniatures is mostly effective wires can occasionally be seen. It is most evident when the aircraft blow apart as bits of wing can be seen swinging on the wire after the explosive detonation.
The biggest problem with the miniature effects is the low frame rate of the filming. None of the shots are filmed at a high enough frame rate. The explosions are so fast that the balsa models are there one frame and in splinters the next. Even the flying shots needed to be filmed at a faster rate as the models on their wires swing and wobble at too high a frequency to be believed.
It has been suggested that the bomb run sequences from this film were in part the inspiration for the Death Star bomb run in Star Wars along with those in The Dambusters 1955.
