I figured not getting a subfreezing high in DJF (yes, I mean 31 and under) would be more common at DCA than BWI. Other than 31/32, the only other one I found was 97/98 (which was tied for 8th hottest winter overall). But there may be others, I can't check them all.
Some of BWI's hottest winters and the first subfreezing high they had (also included the super nino of 72/73 out of curiosity, even though its way down on the list at 25th hottest):
32/33 - 12/16 (23 degrees)
48/49 - 12/26 (25 degrees)
49/50 - 2/26 (31 degrees)
72/73 - 12/17 (30 degrees)
01/02 - 12/30 (30 degrees)
11/12 - 1/4 (31 degrees)