Date: 201206
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZ
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Workload; Confusion; Communication Breakdown; Distraction
Second reporter function: Captain; Pilot Flying
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Company Policy; Human Factors; Airport; Procedure
Primary problem: Procedure

Narrative: Event took place on the first flight of the pairing with a new Captain. We began our flight on time and pushed expecting a normal taxi and takeoff on 12R per the ATIS. We were given clearance to 'Taxi Z; D; to 12R; hold short 12L' by Ground. The Captain commented how unusual this clearance was for us. As I was checking our taxi route and as we approached D we were switched to Tower. Immediately after switching the radio; Tower called before we checked in and cleared us to cross 12L; 17; and line up and wait Runway 12R Intersection D. The Captain called for the Before Takeoff Checklist as we cleared 12L and approached 17 and 12R at D. This caused some confusion for me as it was not our normal taxi clearance for this runway and it was the first time I had taken off from 12R at D. While completing the Pre Takeoff Checklist; crossing 12L and approaching 17 and 12R on D; Tower called and gave us the Intersection 12R at D distance remaining information. I acknowledged this transmission; and finished the checklist as we taxied onto 12R at D. Knowing this was a confusing situation; I took extra care to verify 12R as the Captain turned onto the runway for takeoff where 12R and 17 crossed. The Captain turned to me and asked if I was ready and I responded affirmatively. Looking back; this was the critical point were the workload was high; we were both distracted with several events at once; and we miss-interpreted our last call from Tower as having included our takeoff clearance as we expected. We started the takeoff roll normally. Sometime after passing 80 KTS; we heard Tower call for another Company aircraft on final for Runway 04 to 'go around.' I looked right and realized we had just initiated takeoff without clearance. We continued in lieu of a high speed abort and completed the takeoff. This was a classic example of several events happening at once and not recognizing the increasing risk for error as I became task saturated due to unfamiliar circumstances. Better situational awareness would have prevented this mistake by recognizing the risks piling up: unfamiliar taxi instructions; confusing runway layout; lack of comprehension; ATC instructions during the second call from Tower; and verifying the actual takeoff clearance with the Captain. When Tower gave distance remaining info; it sounded rushed. Had they repeated our clearance to line up and wait with the amplifying information; we might have caught our mistake as we were distracted and assumed the takeoff clearance was issued. Always use standard phraseology with the other crew member and do not assume they know. Recognize your own warning signs and verbalize them so the other crew member is aware that you are uncomfortable/distracted (like when we state we are 'heads down' and not there to back them up).


Second narrative: [Narrative #2 contained no additional information.]


Synopsis: B737 flight crew; given a line up and wait clearance at a runway/taxiway intersection; misinterpreted the Local Controller's distance remaining advisory as clearance for takeoff and initiated takeoff. Controller issued a go-around to traffic approaching a crossing runway.