B737-800
Flight Crew Last 90 Days 137; Flight Crew Total 4782; Flight Crew Type 4782
Confusion; Troubleshooting; Distraction
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Other / Unknown
Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
All preflight and subsequent checks conducted were normal according to FM (Flight Manual). FO (First Officer) was flying pilot. After liftoff; he called for gear retraction and I proceeded with the action. However; after lever movement to the up position; a gear disagree condition became obvious with lights display; feel; and sound. It goes without saying that very serious concerns about the possibility of the inability to obtain terrain clearance; even without an engine failure; was a major factor to consider. Immediate steps were taken to successfully remedy this situation and condition. ATC (Air Traffic Control) was initially notified of the problem; and then advised once it was resolved; so we could continue without any further associated problems or similar conditions. Dispatch; Maintenance Control; and Maintenance were notified and a ELB (Electronic Logbook) report made before and upon arrival.The Manual gear extension access door was found with a very slight gap (smaller than what shows in Ops Bulletin 1-XX-XXX) and indistinguishable from above and looking normal. It is believed that on rotation the FO's flashlight may have come out of his pocket and may have landed on that door and caused it to create this condition; which in my previous 9;500+ in type I have never experienced.
B737-800 flight crew reported being unable to retract landing gear after takeoff.