For those of you having trouble downgrading to the older 3.1.3 firmware or even older versions of iPhone firmware, this tutorial is for you.
This tutorial fixes the problem – This device isn’t eligible for the requested build

I walk you through adding a server IP to your host file to allow you to use the older software.

See my other video on how to downgrade from 4.0 to 3.1.3 and check out the video after the break.