Nobody said anything about a nonstop. WN can get you from MHT to SEA via BWI and MDW. NW will do MHT-DTW-SEA, DL MHT-CVG-SEA. If the OP is price-conscious he's not too worried about nonstops versus connecting flights anyway. (He should not automatically conclude that WN is cheapest. That's what they want you to think, but I've found some very good fares on this route in the $220 range from NW and DL.)
If the OP is on the south side of Boston, WN serves Providence too.