SIN-NRT-IAD 45 minute connection time?

I'm considering flying to/from SIN via NRT. On the return, it shows a 45 minute connecting time. The flight from SIN lands at 3:10pm and the flight to IAD leaves at 4pm. Is that even a feasible connection? I know if I hustle, I can make it through NRT security in 20 minutes. But that's really tight. Does UA hold flights for connecting passengers?