sentences:  On the other hand, Soviet officials tended to order the "filthy pictures" applied to the aircraft removed or painted out.The B-17s remained in service until 1948, when the Tupolev Tu-4 began to arrive at operational squadrons.Beginning in 1952, Republic of China (under the guise of the CIA's Civil Air Transport (CAT) and Technical Research Group (TRG) organizations, operated a number of "enhanced" B-17s (with as many as 14 crewmembers at a time) on surveillance and related flights of mainland China.