Date: 201708
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Landing Gear
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

Narrative: Taxied out for departure on a clear; calm night. Confirmed flaps were in the 1 position on the Before Taxi checklist as well as the Before Takeoff Checklist. I also have my own personal checklist that I run out loud to myself after we take the runway. 'Clock is running; autothrottle is on; flaps and trim in the green.' Before Takeoff Checklist complete; did a throttle burst. No horn. Thinking back on it now; I am absolutely sure we had the flaps set properly. But at the time; we questioned it. Began the takeoff roll. Takeoff thrust set. Once again; no horn. The takeoff was completely normal. V1; rotate and I pitch to approximately 20 degrees to attain the proper climb speed. I called 'landing gear up.' The FO (First Officer) complied and I was fixated on the pitch and airspeed as we climbed out. Seconds after the gear call I noticed the VREF (Reference Speed) +15 white bug was missing as were all of the magenta flap numbers. I initially thought we didn't put the speeds in the FMC. Then as I climbed out I noticed the amber Minimum Maneuver Speed bar went WAY past our current speed of 155 knots and the 'Airspeed Low' alert sounded. I was very confused because we were at least V2+20 and our pitch was not excessive. I thought it was 'impossible' and for the next 30 seconds there was complete confusion. We had normal pitch and normal airspeed. I maintained control of the aircraft but had trouble analyzing what was happening. Seconds after the 'Airspeed Low' alert; the FO said 'the flaps are up.' I looked at the flap indicator and sure enough it read 'zero.' We were well below 1000 ft AGL. Confused; I immediately reduced the pitch and was seriously concerned about lowering it too low as to hit an obstacle or sink even lower so I limited the pitch change since the aircraft seemed to be flying at that point. I didn't want to make huge changes to aggravate whatever situation we were in; which I was still unsure. At that point the FO extended the flaps to 1 and at first I wasn't sure if any changes should be made but it seemed to improve our situation. We climbed to flap retraction altitude; increased our airspeed and retracted the flaps. I turned on the autopilot to reduce the workload so I can assess what just happened. 'Did we just take off with no flaps?' I asked the FO. 'I don't know' was the response. And neither did I to be honest. Still trying to figure it out; I wanted to make sure we were away from the ground with plenty of airspeed. So I leveled off at the assigned altitude of 3000 ft and accelerated to 250 knots. After the level off; we felt a rumbling on the airframe. Thinking that the aircraft is damaged or that there is still a flap issue; I looked at the leading edge of the wing and the slats were retracted. I then looked up at the overhead panel and the slats indicated 'up.' Aircraft is still rumbling and I don't know why. Confused; frightened; stressed and seeking answers; neither one of us realized that the gear was still down at 3000 ft and 250 knots. The FO said 'the gear is still down;' and raised the gear at 250 knots. The rumbling stopped. We continued uneventfully. I wrote up the gear exceedance and called Maintenance Control to report it.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported confusion with the flap setting on takeoff that led them to forget to retract the landing gear.