The Washington State Legislature’s web site has historical materials dating all the way back to 1854.
Legislative history for bills introduced from the most recent 3 bienniums can be researched from the Legislature’s Bill Information page: https://apps.leg.wa.gov/billinfo/
For information from 1991 through the current year, visit the Legislature’s Detailed Legislative Reports page. From there, use the Biennium drop-down menu on the right-hand side of the page to select the year of the bill, then type the bill number into the Bill Number field on the left-hand side of the page. Click the Continue button to proceed. Pending legislation can be tracked here as well.
For information from 1985-present, click on the Search (Documents) link in the Detailed Legislative Reports page and scroll to the bottom. This search feature allows you to look for bills, budget bills, bill histories, and amendments from 1985-present, bill analysis reports from 1987-present, and roll call votes from 1991-present, among other things.
Archived materials from the reporting committees (dating back to 1999 in many cases) can also be found on the Legislative Committees page of the Legislature’s web site. The materials can be found under the links for each respective committee.