Since the first and only times it was used 40 years ago, the atomic bomb has been a source of controversy, fear and myth -- and a great deal of misinformation. Much of the history of "the bomb" has been obscured or mistold.

Debate continues, for example, over whether the United States should have invented the bomb or should have dropped it on Japan. Yet, as in much of today's discussion over strategic weapons, all the facts surrounding the development and dropping of those first nuclear weapons have not been understood.

In 1944, the United States originally planned to drop atomic bombs simultaneously on Germany and Japan, according to Manhattan Project officials involved in developing the bomb and previously unpublicized documents from that time.

An island in the Adriatic had been selected as the base for U.S. airplanes in the European operation, according to retired Brig. Gen. Paul W. Tibbets, who commanded the B29 bombing unit and piloted the Enola Gay, the plane that dropped the first atomic bomb on Hiroshima.

By the spring of 1945, however, months before the first bomb was ready, the defeat of Nazi Germany was assured, so plans were limited to Japan.

But the U.S. military establishment, starting with Chief of Staff Gen. George C. Marshall, doubted the bomb would work. Conflicting and competitive plans for ending the war in the Pacific were put forward by the separate military services; the bomb was not central to any of them.

Scientists developing the bomb, including the Manhattan Project director, Dr. J. Robert Oppenheimer, believed that 12 to 15 atomic bombs would be needed to force the Japanese to surrender, the project sources said. One scientist proposed that use of the weapon be delayed until the United States had nine bombs, and that all nine be dropped on the same day.

These are among the facts learned from a three-month study of the times when nuclear weapons were used -- and the crises when they weren't -- conducted for The Washington Post and for a CBS documentary to be broadcast July 31.

A review of government documents, many never before released, and interviews with two former presidents, four secretaries of defense and three national security affairs advisers indicate that despite the increase in the number and power of nuclear weapons -- or because of them -- the United States and the Soviet Union have, during these 40 years, drawn back from threats to use them; they have even sought to avoid conventional military confrontations out of fear that such confrontation could lead to a nuclear showdown.

At the same time, the confidence shown by the U.S. military in the late 1940s and 1950s that nuclear weapons would be usable in wartime has been largely replaced by a belief that such weapons are needed primarily to prevent the other side from using them.

This survey of the last 40 years shows that:

*In the post-World War II period, President Dwight D. Eisenhower signed a national security directive that equated nuclear weapons with conventional weapons, thereby allowing the Joint Chiefs of Staff to plan equally for their use and purchase. When Eisenhower asked for specific plans to meet crises in Korea, the offshore Chinese islands of Quemoy and Matsu, and Vietnam using nuclear weapons, such plans were drawn up. But none was approved.

*The United States came closer than is generally known to bombing and invading Cuba in 1962 during the missile crisis. But the presence of nuclear-armed Soviet missiles on the island led the two superpowers to an unpublicized but explicit tradeoff to end the crisis -- removal of Soviet missiles from Cuba in return for withdrawal of U.S. missiles from Turkey.

*Both the Soviet Union and the United States used the movement of nuclear weapons as diplomatic signals of political resolve during the 1973 Israeli-Egyptian war. They have not faced a military crisis in the past 12 years serious enough to warrant nuclear diplomacy.

From the start, secrecy has cloaked important facts about the nuclear age. Even when historical information has been published in books and scholarly articles, it has not been widely circulated. It has been easier, therefore, for myths to flourish. 'Fat Man' and 'Little Boy'

In the 1940s, Manhattan Project scientists developed two types of bombs -- a uranium-based model nicknamed "Little Boy," and a plutonium-type bomb called "Fat Man."

The scientists were so certain that Little Boy would work that they felt no need to test it before it was dropped over Japan. The weapon used the "gun-barrel" approach in which one chunk of uranium was fired down a 5-inch barrel at a uranium target, colliding in a split second to form the correct critical mass of uranium for a fission explosion.

However, because it took six months or more to process enough uranium to make the 132 pounds of weapons-grade material needed for each Little Boy bomb, the scientists believed that another approach was needed to produce the number of bombs they thought would be required to force a Japanese surrender.

Only 12 pounds of plutonium was required to make a bomb, and that amount could be produced each month. But the gun-barrel approach was not feasible for plutonium because of the material's physical characteristics. So the scientists had to develop a more innovative method -- forcing two hemispheres of plutonium together into a critical explosive mass by a timed explosion of 64 charges around the outside of the hemispheres. Because scientists were uncertain that this "implosion" approach would work, they had to test it at Alamogordo, N.M.

After the successful test of this type of plutonium bomb almost exactly 40 years ago -- July 16, 1945 -- an experiment code-named "Trinity," Oppenheimer suggested to his superior, Maj. Gen. Leslie Groves, that they cancel dropping the first uranium bomb on Hiroshima. Instead, he wrote in a telegram on July 19, 1945 that he could use the large amount of nuclear material in Little Boy to build four additional plutonium bombs, so that 15 would be available by Nov. 1, 1945, the date selected for the invasion of Japan.

Groves replied the same day: "Factors beyond our control prevent us from considering any decision other than to proceed according to existing schedules for the time being. It is necessary to drop the first Little Boy and the first Fat Man and probably a second one in accordance with our original plan."

Another suggestion from three Manhattan Project scientists after the Alamogordo test -- also rejected -- was to attach a "super powerful siren" to the measuring devices dropped along with the first bomb over Japan, so that the Japanese would look up. "Nobody within a radius of five miles could look directly at the gadget and retain his eyesight" when it explodes, the scientists wrote, according to a declassified 1945 document.

The initial Manhattan Project targeting studies were done in 1944, when the war against Nazi Germany was still under way. Therefore, the studies contemplated using the weapon in Germany and Japan, which created the need for different tactics.

In a 1944 paper, Dr. William Penney, then a British scientist attached to the Manhattan Project, wrote that different heights for exploding an atomic bomb (referred to as the "gadget") were suggested for the two countries because "there is a significant difference in the blast-resisting characteristics of German towns and those of Japanese towns."

Tibbets said in an interview that he had been told to prepare "for a split operation . . . simultaneous drops in Europe and Japan." But by early 1945, with the war in Europe heading toward conclusion, it was clear that the first weapons would not be available until August, so plans focused solely on Japan.

In selecting Japanese cities, U.S. officials planned to use the weapon more for the terror it would produce than for its military effect. In one high-level meeting, Dr. James Conant, then deputy scientific adviser to President Harry S Truman and later president of Harvard University, suggested that the targets be cities with military installations "with close-in civil populations."

"They wanted a shock action . . . to bring about a surrender," John J. McCloy, then deputy secretary of war, said in an interview. "And that's why they chose a spot that was close to a civilian population."

The scientists were aware of the radiation expected from the bomb, although their understanding of the types of radiation and the short- and long-term health effects was limited.

A declassified memorandum of May 11, 1945, from Oppenheimer to Groves' deputy, Brig. Gen. Thomas F. Farrell, said, "During detonation, radiations are emitted which (unless a person is shielded) are expected to be injurious within a radius of a mile and lethal within a radius of about six-tenths of a mile."

A 1944 targeting document by Manhattan Project scientists suggested that "the possibility of eliminating a large fraction of the fire[-fighting] force of a Japanese town by getting the firemen into the radioactive contamination area to fight fires is attractive and realistic."

The ancient Japanese capital city of Kyoto was originally picked by the target committee in Washington. Hiroshima and Niigata were chosen as secondary targets. Kyoto was selected because it was a historic city and was "occupied by intellectuals whose report of the weapon would have an effect on the Japanese people," according to minutes of the May 10, 1945, meeting.

At the insistence of Secretary of War Henry L. Stimson, who believed that the cultural capital of Japan should be preserved, Truman eliminated Kyoto from the target list. It was replaced by Hiroshima.

From May through July 1945, some scientists and officials tried to delay the bombing of a Japanese city. Some, including Dr. Edward Teller, wanted to stage a demonstration explosion over Tokyo Bay. Others, including McCloy, wanted to warn the Japanese that if they did not surrender a terrifying new weapon would be used against them.

Truman listened to the objections and set up a special committee to review plans for the bomb.

Oppenheimer became one of the strongest advocates for dropping the weapons on Japan without advance notice. His reasoning, accepted by most Los Alamos scientists at the time, was that there was no guarantee that either bomb would work and no feasible way to stage a demonstration that the Japanese military would permit to take place.

Truman was much affected by an argument put forth by British Prime Minister Winston Churchill at Potsdam after word of the successful Trinity test arrived, according to several sources. The British war leader argued, according to his memoirs, that "one or two shocks" could end the war by giving the Japanese "an excuse to save their honor and release their military from the samurai obligation to fight to the death."

The "psychological impact of the weapons were far greater than their military consequences," Paul Nitze said recently. Nitze, 78, now special adviser to Reagan on nuclear arms control, was a member of the strategic bombing survey during World War II and one of a handful of specialists called in by Truman to analyze competing military plans to end the war with Japan. In the summer of 1945, Nitze said, only the scientists regarded the atomic bomb as the way to end the war. The Army wanted to invade; the Navy proposed to blockade Japan into surrender; the Air Force suggested destroying it through conventional, nonstop, heavy bombing. A long-term, joint bombing and submarine blockade approach was rejected by Marshall and a majority of the military commanders in favor of a Nov. 1 invasion of the southern Japanese island, Nitze said.

Truman authorized the dropping of two bombs after Aug. 3, the deadline set by the Allies' Potsdam statement calling on Japan to surrender unconditionally. The Day of Destruction

The uranium projectile for Little Boy arrived on Tinian Island in the Pacific about 2,000 miles from Japan on July 26, aboard the cruiser USS Indianapolis. The two target pieces of uranium arrived aboard air transport planes on July 28-29. The Little Boy bomb was assembled and ready by Aug. 2. The weather, however, did not clear until Aug. 5.

The weapon was loaded aboard the Enola Gay that afternoon at 2 p.m. After a midnight final briefing, the crews for the first mission assembled by their planes.

"There amid brilliant floodlights, their pictures were taken and retaken by still and motion picture photographers as if for a Hollywood premiere," wrote Norman F. Ramsey, the senior scientist on Tinian, in an official history of the episode.

At 2:45 a.m. on Aug. 6, they took off. Six and one-half hours later, at exactly 9:30 1/2 a.m., according to a log kept on the plane, the bomb was dropped -- only one-half minute later than planned.

Three B29s were on the raid. One carried the bomb; the second carried scientific instruments that were dropped at the same time as the bomb to measure its explosive power; the third carried a "Fastex" experimental camera to photograph the fireball.

Similar three-plane missions had flown over Japan in the previous two months without dropping anything more powerful than one small high-explosive charge in a pumpkin casing, a test-bomb casing shaped like a Fat Man weapon and painted orange. That tactic was designed by Tibbets to train his pilots to get over the target and, at the same time, lull the Japanese into thinking they had nothing to fear from such formations.

With clear skies over Hiroshima on Aug. 6, no planes came up to intercept the Enola Gay and its two companion aircraft.

Tibbets recently described his reaction as he looked back after dropping the bomb and saw "what had been the city down there was nothing but a black, boiling pallor of . . . I called it tar. But it was smoke, debris, dust, and everything. It was bubbling, just boiling."

The airborne measuring devices dropped from the scientific plane showed a reading of about 14 kilotons, according to Dr. Harold Agnew, who was aboard the plane. When word of the successful attack was released in Washington, Truman said it was equal to 20 kilotons -- a statement that forced the group on Tinian to classify their own findings, because they showed a lesser blast than the one reported by the president.

After the raid on Hiroshima, both the scientists on Tinian and the military commanders in the Pacific wanted to drop the second bomb before bad weather closed in. With bad weather approaching, the date of the second drop was advanced from Aug. 20 to the 11th, then to the 9th.

"The advancement of the date by two full days introduced a large measure of uncertainty," Ramsey wrote. For example, the first test drop of a complete Fat Man-type with all its implosion systems -- but without its nuclear components -- did not occur until a flight from Tinian on Aug. 8, the day of the final rehearsal flight for the B29 crew that would carry it and the day before the mission.

"Without doubt," wrote retired vice admiral Frederick L. Ashworth, who was in charge of the bomb on the raid, "this was the shortest time between development and operational use of any weapon in ordnance history."

On Aug. 7 and 8, Gen. Curtis LeMay, head of the Pacific bombing command, had his conventional bombers drop leaflets on raids over Japan that said: "America has developed an atomic bomb more terrifying than anything anyone has ever been able to make before . . . . One of these bombs was dropped on Hiroshima . . . . America has many of these bombs. They hope that you will entreat the Emperor to end this war before they must use them."

The Japanese military, however, had allowed only limited information about the Hiroshima destruction to circulate in Japan. Nitze and several of the Manhattan Project scientists said their interviews with Japanese officials after the war confirmed that the military intended to fight on, believing that only one U.S. bomb of the type dropped on Hiroshima existed.

The second mission was marred by a series of mishaps. Its primary target was Kokura, site of a Japanese arsenal. The three planes took off late from Tinian on the morning of Aug. 9. They planned to rendezvous 1,500 miles later over a small island south of Japan.

Before takeoff, the plane with the bomb, Bocks Car, found that the pump on its auxiliary gas tank was clogged. Because bad weather was closing in, however, it was decided to take off without making repairs. At the rendezvous point, the scientific plane appeared but the photo aircraft did not. (The pilot of the photo plane, a last-minute substitute, did not make it to the correct assembly point and never made it to Nagasaki.)

After circling for 50 minutes, maintaining required radio silence and waiting for the third plane, Ashworth and the B29 command pilot, Major Charles W. Sweeney, aboard Bocks Car agreed to head for the assigned target, Kokura.

According to Ashworth, "55 more minutes of precious fuel" were used up making three unsuccessful bombing runs over Kokura. Each time smoke and haze covered the target and the plane was under orders to drop the weapon only by visual rather than radar sightings. At that point the two agreed to go to the secondary target, Nagasaki, a decision, Ashworth recorded, that "was punctuated with antiaircraft fire and Japanese fighters being vectored to our altitude." They had only enough gasoline to make one run over Nagasaki and still have fuel to make it back to Okinawa. Had they not dropped the 10,000-pound weapon, they could not have made it even that far.

Sweeney said recently that he never doubted he would drop the bomb, recalling that he had been told by one of his commanders that the bomb cost $2 billion to build and the airplane far less.

Nagasaki, too, was clouded over but Ashworth ordered the bomb dropped by radar, against his standing orders. With just seconds to go, however, the clouds broke and the visual bombsight was used. Ironically, the bomb missed its target, which was, according to Ashworth, "the city center," and instead hit one mile away but much closer to the supposed military target, a torpedo factory.

Even after the second bomb was dropped, Groves and the scientists were uncertain that Japan would surrender. Assembly of the nonnuclear parts of the third bomb began on Tinian. However, Truman had not approved shipment of the nuclear material, although Tibbets said recently that it had gone as far as California.

Japanese Emperor Hirohito visited Nagasaki and later presided over a cabinet meeting where he said he would defy precedent and sue for surrender without the unanimous approval of his ministers.

On Aug. 10, Groves in Washington reported to Marshall that the next Fat Man bomb would be "ready for delivery on the first suitable weather after 17 or 18 August," according to declassified documents. That was one week earlier than orignally planned, Groves wrote.

On Aug. 14, Tibbets sent seven B29 bombers over Japan, to cities such as Korona and Nagoya, to drop the same type of pumpkin test bombs that preceded the earlier two atomic bomb raids.

That same day, Aug. 14, the emperor's surrender message was announced.