Do I need to apply for a transit visa?

I have a US passport. I fly directly to Vietnam. My return flight itinerary is

Ho Chi Minh City, VM (SGN) to Beijing, CH (PEK)

Departing
Tan Son Nhut Airport
Sat, Jun. 14, 2014 at 5:40 AM
Arriving
Beijing Capital Intl Airport
Sat, Jun. 14, 2014 at 11:30 AM
Air China

Layover Time: 2h and 30min

Beijing, CH (PEK) to Los Angeles, CA (LAX)

Departing
Beijing Capital Intl Airport
Sat, Jun. 14, 2014 at 2:00 PM
Arriving
Los Angeles Intl Airport
Sat, Jun. 14, 2014 at 11:00 AM

Will I need one for this short amount of time ?