How to unfold a belt based on offset surface

I don't know how exactly to do it in Solid but in Inventor you have to make a cut (just like i think you did in the top middle of your profile) and proceed a command to unfold it, make sure the thickness of your part matches the rule thickness set.

Convert the part to sheet metal and then it will allow you to unfold it into the flat pattern you are looking for.

http://help.solidworks.com/2014/english/solidworks/sldworks/hidd_feat_sm_fold_up_down.htm

This is the type of message I got while using insert Bends.

Why are you trying to flatten such an easy piece of geometry? It is just a simple rectangle when flattened....You just need to measure the lenght of the inner loop (tools/evaluate/measure) and draw a simple rectangle as flat surface for cut .