I've been asked by a Taiwanese aviation historian about which air arm and unit logged the longest documented B-17 mission. The Republic of China Air Force (RoCAF) operated a handful of B-17s for agent-dropping missions into mainland China, among other things.
On 26 February 1960, one B-17 from the Technical Research Group (official name of No. 34 Sqdn) flew a round trip to Yunnan in southern China, logging a total of 19 hours and 10 minutes in the air.
There are three CIA/RoCAF B-17 losses since 1950 listed HERE.
26/5/54: Special Ops Unit B-17. Lost to PLA AAA, Fujian.
22/6/56: Special Ops Unit B-17. Lost to PLA MiG-17, Jiangxi.
29/5/59: 34 Sqdn B-17 "835". Lost to PLA Mig-17, Guandong.