Do I need a transit visa for leaving the airport for less than 8 hours?

I am travelling to Mumbai, India from Houston, USA on 28th December, 2015. I have a layover of 14 hours and 40 minutes at the Beijing airport. I was planning on exploring the city in that time. Do I need an entry permit to leave the airport? And if yes, how much will it cost? And can I get it on arrival?

Thanks!