Comment by Merad

I don't know about the B-29 system in particular, but gyroscopic gunsights used on most WW2 fighters were fairly straightforward. First, you set a selector on the sight to the wingspan of your target (usually there would be presets for common enemy aircraft). Then you twist a knob (typically on the throttle) that adjusts a ring in the sight until it matches the size of the target - this establishes the range. Finally a gyroscope provides input on how your aircraft is moving, and since you're typically maneuvering to follow your target this is an approximation of the target's movement.

There's a lot of approximation involved, but generally it's good enough to hit an airplane sized target, especially on US fighters where you typically have 4-8 machine guns spitting out 50+ bullets per second in a shotgun like pattern.

Some relevant links:

http://i651.photobucket.com/albums/uu236/ken1926/P-51DK_03.j...

http://www.lonesentry.com/blog/wp-content/uploads/2010/09/ta...

https://upload.wikimedia.org/wikipedia/commons/d/d7/P-47_gun...