The WHO draft report, which was obtained by The Associated Press news agency, pointed to serious errors by an agency designated as the international community's leader in coordinating response to outbreaks of disease.
The document - a timeline of the outbreak - found that WHO, an arm of the United Nations, missed chances to prevent Ebola from spreading soon after it was first diagnosed in Liberia, Sierra Leone and Guinea last spring, blaming factors including incompetent staff and a lack of information.
Its own experts failed to grasp that traditional infectious disease containment methods would not work in a region with porous borders and broken health systems, the report found.
"Nearly everyone involved in the outbreak response failed to see some fairly plain writing on the wall," WHO said in the report.
"A perfect storm was brewing, ready to burst open in full force."
The agency's own bureaucracy was part of the problem, the report found.
It pointed out that the heads of its country offices in Africa are "politically motivated appointments" made by the WHO regional director for Africa, Dr. Luis Sambo, who does not answer to the agency's chief in Geneva, Dr. Margaret Chan.
Dr. Peter Piot, the co-discoverer of the Ebola virus, agreed that WHO acted far too slowly.
"It's the regional office in Africa that's the front line," said Piot, interviewed at his office in London. "And they didn't do anything. That office is really not competent."
WHO declined to comment on the document, which was not issued publicly, and said that Chan would be unavailable for an interview.