Date: 201507
Local time of day: 1201-1800
Location: DCA.Tower, DC
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight phase: Initial Approach
Airspace: Class B DCA
ATC / advisory: Tower DCA
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Situational Awareness; Distraction
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Arriving into the DCA area; we were cleared for the MOUNT VERNON Visual Approach Runway 01 and were told to maintain 170 knots to a five mile final. We were then switched to DCA Tower frequency at about seven miles. We checked-in with DCA Tower about six miles and they cleared us to land and told us there would be one departure before our arrival. We began further slowing our approach speed and continued configuring gear and flaps. Tower then cleared another carrier 737 for takeoff with us right around or slightly inside of four miles. The other carrier flight seemed to move slowly onto the runway and it was starting to feel a little tight and this became a distraction. Tower then told the other carrier that there was an aircraft on two and one half mile final; cleared for immediate take-off. The other carrier responded that they were 'cleared for take-off'. A few seconds elapsed and it did not appear that they were rolling yet and Tower said; 'Other carrier XXX; aircraft on two mile final; we need you to move'. [Other carrier] responded: we're rolling. Throughout this exchange of transmissions; we were convinced that we would be sent on a go-around. With all the distractions; the FO (First Officer) and I; after discussing the incident later; cannot remember if we ever accomplished the Before Landing Checklist. At any rate; at approximately 300 ft AGL; we received the warning: 'Too Low Flaps'! We immediately saw that the flaps were at 25 degrees as opposed to the briefed 30 degrees. The F/O instinctively positioned the flap handle to 30 degrees as I instinctively directed a go-around. On go-around the Tower questioned the reason and we responded that we did not have a stabilized approach. We continued on for an uneventful visual approach and landing.In my mind; I was already thinking go-around and I was plotting my maneuver to make sure we respected the prohibited areas across the river. In hindsight; my focus needs to be the completion of the task at hand; which was; the approach and the before landing checklist. The FO and I honestly cannot remember if I called for flaps 30 and he only put them to 25 or if I never called for flaps 30 at all. We also cannot honestly remember if we ran the checklist and both just failed to recognize the improper flap position. Either situation is disconcerting and opens our eyes to the dire threat of distractions in the cockpit. This never happened to me before and I vow that it won't happen to me again.


Second narrative: [Report narrative contained no additional information]


Synopsis: B737-700 flight crew reported they failed to set landing flaps because of a distraction; and executed a go-around at 300 ft AGL.