Five months before the Bay of Pigs invasion, the CIA task force plotting to overthrow Fidel Castro concluded that the invasion was ''unachievable'' as a covert paramilitary operation, according to a newly discovered unclassified document.

Indeed, historians have documented individuals expressing doubts at various times before the ill-fated mission.

But the document, a 300-page internal CIA history, reveals for the first time that the architects themselves foresaw failure during a Nov. 15, 1960, meeting to prepare a briefing for President-elect John F. Kennedy and that they recorded it in a memo.

''There will not be the internal unrest earlier believed possible, nor will [Castro's] defense permit the type [of] strike first planned,'' say notes of the meeting, according to the official CIA historian, Jack Pfeiffer. ''Our second concept (1,500-3,000) man force to secure a beach with airstrip is also now seen to be unachievable, except as a joint Agency/DOD [CIA/Pentagon] action.''

Historians say it is unclear whether CIA Director Allen Dulles and his deputy passed this assessment along three days later, at Kennedy's post-election national security briefing in Palm Beach -- and whether changes were made as a result of the finding. But, with Kennedy's blessing, the so-called ''unachievable'' CIA-only second concept went forward five months later, on April 17, 1961 -- with devastating consequences.

Castro's forces defeated the CIA-trained and backed brigade in less than 72 hours; about 114 men were killed, and more than 1,100 forces were captured and held until the United States traded $53 million in food and medicine for their freedom.

Afterward, military experts blamed the fiasco on a decision to withhold air support, a bad choice of location, and U.S. refusal to provide U.S. troops as reinforcements. ''The CIA knew that it couldn't accomplish this type of overt paramilitary mission without direct Pentagon participation -- and committed that to paper and then went ahead and tried it anyway,'' said Peter Kornbluh, senior analyst at the National Security Archive and author of Bay of Pigs Declassified, who said the disclosure is new.

Even Pfeiffer, the CIA's official Bay of Pigs historian, noted the paradox in his long-classified Volume Three of the history, on the Eisenhower years: 'How, if in mid-November 1960 the concept of this 1,500-3,000 man force to secure a beachhead with an airstrip was envisioned by the senior personnel . . . as 'unachievable' except as a joint CIA/DOD effort, did it become 'achievable' in March 1961 with only 1,200 men and as an Agency operation?''

Both Kornbluh and Villanova University political scientist David Barrett were struck -- separately -- by the revelation while reading Pfeiffer's report, which Barrett discovered in June in a box marked ''Miscellaneous'' at the National Archives.

Pfeiffer, who died in 1997, wrote it at the CIA in the late 1970s from classified records and interviews with architects and operatives.

It reads like a 300-page chronicle of mission creep and misadventures in the embryonic effort to oust Castro -- from proposals to stage dirty tricks to early talks in Miami and New York between CIA agents and American executives on how to foil the young Cuban revolution. WAS KENNEDY TOLD? In it, Pfeiffer wrote of the Nov. 15, 1960, session of the CIA task force code-named Western Hemisphere Branch Four (WH/4), which met to prepare a summary for the deputy director for plans, Richard M. Bissell Jr., to help Dulles brief Kennedy on foreign affairs.