The driest summer, which we'll measure from June 1 to Aug. 31, was in 1919, when we only received 0.65 inches of rain for those three months. That was when records were kept in downtown Seattle. For the current location (Sea-Tac Airport), which has been there since 1945, the record is 0.84 inches in 1987. 1914 was also noteworthy in that we only received 0.02 inches between July 1 and Aug. 31.
Seattle has never gone a calendar month with zero rain, but there have been many months when we've only received a trace -- where it rained but not enough to measure.
This summer is still on par to be quite dry, only having received 0.53 inches since June 1.