Will a DC motor actually produce voltage if it is spun in reverse?

It'll make voltage if you spin it forwards, too. The only difference among a DC motor and a DC generator is what you use it for.

A long time ago, cars had DC generators on them instead of AC alternators. When you changed a generator, you had to "polarize" it so it would make electricity. You hooked it up to your battery and let it run, as a motor, for a few seconds. Don't try that with an alternator because you'll break it.

If you're looking for a neat little elementary-school science project, you can use little Radio Shack electric motors as generators for things such as windmills, water wheels, dynamos turned by bicycle tires..