Transit visa for 72 hours at Xi'an Xianyang International airport

I am a US citizen flying from USA to Xi'an with a change in plane in Shanghai. I want to take a 72-hour stopover in Xi'an so that I can visit the Terracotta Warriors excavation site. I will be flying out of Xi'an to Kolkata, India with a plane change at Kunming. I will not take any stopover at Shanghai or Kunming though - only flight change. I have valid US passport. Am I eligible for 72-hour Free Transit? How do I get it?