The US National Transportation Safety Board has released the initial findings of its investigation into the incident on an Alaska Airlines plane in January.
The report states that the unused door was missing four key bolts.
Responding to the report, Boeing said it was responsible for what happened.
Boeing President Dave Calhoun said in a statement: "This should not happen on an airplane leaving our factory. We must do better for our customers and their passengers. We are implementing a comprehensive plan to strengthen quality and the trust of our stakeholders."
The incident occurred minutes after Alaska Airlines Flight 1282 took off.
The panel covering an unused emergency exit, known as a door stopper, suddenly exploded, leaving a gaping hole in the side of the plane's main fuselage.
Missing bolts caused the door panel to dislodge and detach from the plane, the report states.
The incident happened a few minutes after the plane took off from Portland International Airport.