B737-700
Flight Crew Last 90 Days 203; Flight Crew Type 12000
Distraction
Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Aircraft; Airport; Procedure; Human Factors; Company Policy; Chart Or Publication
In the process of starting #2; the first engine; the First Officer (FO) commented that one of our airplanes was holding out to taxi into our alleyway.  I almost said; 'Don't let that change our pacing or flows;' but didn't so I wouldn't interrupt [the] start and flow process.  Ground Controller issued over a minute's worth of taxi instructions without interruption; which interrupted the FO's After Start flows.  [FO] became focused on getting in a request for taxi....  We taxied straight out to the runway and ended up about number 3 in line for takeoff.  Just before setting brakes; I did the throttle burst and started my pretakeoff flow; and the Takeoff Warning Horn came on.  I immediately glanced at the flaps and saw the flaps were still up.  I told the FO we needed to get the flaps to 5; takeoff flaps. [The FO] looked at the TCAS and saw Standby and said; 'We forgot to run the Before Taxi Checklist.'  We stopped; set the flaps to 5; ran the Before Taxi and Pre-Takeoff Checklists; and took off without further incident.This is the first time this has ever happened to me.  I allowed myself to become preoccupied as a Pilot Monitoring (PM) and descended into the same frame of reference as the FO due to change of pace and distractions. Thank heavens we added the throttle burst to our pretakeoff flows years ago.  And; I remember thinking it will sure be nice when we get a flap check added to the Before Takeoff Checklist.   I have long said many of our serious checklist omissions are caused by interruptions at precisely the wrong time causing us to forget running an entire checklist.  Today; I bore the prediction true.
A B737-700 flight crew failed to complete the Before Taxi Checklist due to distractions and discovered the flaps were not set for takeoff when the Takeoff Warning Horn sounded.