Someone is to do something

Billy was to ensure that the computer was packed safely.

I often see this "to --" (infinitive?) form used in sentences. And I've been confused about what does this form mean, and how it is used. In the case of the above sentence, if it were up to me, I would just simply say: Billy ensured that.. OR Billy has ensured that...

Can anyone explain this to me once for all, or direct me to a website that I can read more about this form (someone is to do something)?