Arriving at PEK on United, from US (ORD) at 315pm. Domestic flight with Air China at 515pm to XIY.

It's international to domestic transfer in T3. Generally speaking, the transfer time is at least 2 hours. Thus your time is tight. If possible, you'd better to change the schedule of your next flight.
BTW, the transfer procedures are the following:
Inspection & Quarantine, Exit and Entry Frontier Inspection - Baggage Claim - Customs Inspection - Going through Transfer Formalities - Security check - Waiting & Boarding