Is a visa required for US citizen

Hi , I am a US citizen going to Bali and am stopping off in Beijing for 72 hours.
I am arriving on a ticket from Detroit to Singapore on DL and then a separate ticket on Hong Kong Air departing to Beijing. On the return to Detroit I am continuing on my DL ticket- Can I just get a transit type at the Beijing Airport? Do I have to do anything prior to my arrival in Beijing?