A B757-200 WINGTIP STRUCK AN EMB170 AS THE B757 ATTEMPTED TO ENTER THE RWY FOR TKOF IN DARKNESS AND RAIN.

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IT WAS RAINING IN WASHINGTON. WE EXPERIENCED A WX GATE HOLD FOR DEPS TO ZZZ2. WE STARTED A SINGLE ENG AND TAXIED OUT FOR DEP ON RWY 19. WE WERE #3 FOR DEP. AN ACR X EMB170 WAS #1 AND AN ACR X B737 #2. THE ACR X B737 INFORMED THE TWR OF AN OPEN ACCESS PANEL OPEN ON THE BOTTOM AFT OF THE EMB170 FUSELAGE. AT THIS TIME WE STARTED OUR SECOND ENG. THE EMB170 REQUESTED TO RETURN TO THE GATE. THE TWR INFORMED THEM THAT THEY WOULD HAVE TO WAIT DUE TO ARRS AND DEPARTING TFC THAT BLOCKED THE WAY FOR THEIR RETURN. THE TWR THEN QUICKLY CLRED THE ACR X B737 INTO POS AND FOR IMMEDIATE TKOF. THE TWR THEN INSTRUCTED US TO CLOSE THE GAP IN THE RUN-UP AREA THAT WAS CREATED BTWN US AND THE EMB170 AFTER THE DEP OF THE B737. THE CAPT COMPLIED WHILE I MADE THE PREDEP PA; RAN THE AFTER START AND BEFORE TKOF CHKLISTS; AND CONTACTED THE FLT ATTENDANTS ON THE INTERPHONE FOR ACKNOWLEDGEMENT THAT THE CABIN WAS READY FOR DEP. THE TWR THEN INSTRUCTED US TO TAXI INTO POS AND HAVE THE PWR UP AND READY FOR IMMEDIATE DEP. THE CAPT STARTED TO TAXI THE ACFT. I LOOKED OUT THE WINDOW ACROSS THE COCKPIT AND THOUGHT WE WERE FAIRLY CLOSE TO THE EMB170. AT THE SAME TIME; THE CAPT EXPRESSED THE SAME CONCERN AND BEGAN TO CORRECT BACK TO THE R. THE TWR INTERJECTED THAT WE WERE NOT MOVING AS INSTRUCTED AND THAT WE NEEDED TO EXPEDITE ONTO THE RWY. I RESPONDED THAT WE WERE MOVING INTO POS. AT THAT MOMENT WE FELT A SLIGHT BUMP; AS IF WE TAXIED OVER A CRACK IN THE TXWY. THE CAPT THEN COMMENTED THAT WE HAD MADE CONTACT WITH THE EMB170 AND THE TWR RPTED THAT THE EMB170 RPTED THAT THEY HAD BEEN BUMPED. WE ALSO RECEIVED A CALL FROM THE FLT ATTENDANTS THAT THEY HAD PERCEIVED THE SAME THING. I CALLED BACK TO DCA OPS TO INFORM THEM THAT WE WOULD BE RETURNING TO THE GATE AND THAT WE WOULD NEED MAINT TO MEET US. WE THEN INFORMED THE TWR THAT WE WOULD RETURN TO THE GATE. I WENT OUTSIDE TO POSTFLT THE AIRPLANE AND CONFER WITH MAINT. THERE WAS NO VISIBLE DAMAGE THAT WE COULD SEE FROM THE GND OR THAT MAINT COULD ASCERTAIN FROM THE SCISSOR TRUCK. AT THE REQUEST OF MAINT; I TURNED ON THE HYDS AND EXTENDED THE FLAPS TO 15 DEGS; THEIR POS WHEN THE INCIDENT OCCURRED. WE WENT BACK OUTSIDE THE ACFT. ONCE OUTSIDE WE WERE THEN INFORMED THAT THE COMPANY WANTED US AWAY FROM THE SCENE. OPS PERSONNEL HAD US GATHER OUR BAGS AND TOOK US TO A CONFERENCE ROOM. I HAD TAKEN THE LOGBOOK OUT AND PLACED IT ON THE GLARE SHIELD IN ORDER TO ENTER A WRITE-UP BUT WAS UNABLE TO COMPLETE THAT TASK BEFORE WE WERE INSTRUCTED TO LEAVE THE ACFT. SUPPLEMENTAL INFO FROM ACN 654366: OUTSIDE FACTORS INCLUDED NIGHT OPS AT A BUSY ARPT AND LIGHT RAIN THAT REQUIRED WINDSHIELD WIPER OP. SIDE WINDOWS NEEDED FOR TAXI CLRNC HAD WATER ON THEM; REDUCING THE VISIBILITY.