The flight began at 4 p.m. on May 15 at Clovis Army Airfield in New Mexico, a training base for B-17s involved in preparations for Operation Crossroads -- the planned nuclear weapons tests at Bikini Atoll. Lieutenant Warder Skaggs was at the controls, with three other crewmen and 12 passengers aboard. The route took them southwest to Davis-Monthan Field near Tucson, then west to Mines Field near Los Angeles, where several passengers got off. From Los Angeles, the B-17 turned north for the final leg to Hamilton Army Airfield in Marin County. The plane was due at 1:17 a.m. It had the fuel capacity for a nonstop transcontinental flight and had topped off its tanks near Los Angeles, just 400 miles south. Running low on fuel should have been impossible. Yet that was one of the official causes listed after the investigation.

The tragedy turned on a single misunderstanding. When the crew radioed Oakland to report they were lost and low on fuel, they believed they were somewhere over the East Bay. The Oakland operators believed the same thing. In reality, the B-17 was directly above Hamilton Field -- its destination -- obscured by a fog bank that had erased every visual reference. Had anyone realized the plane's true position, a simple descent through the overcast would have put it on Hamilton's runway within minutes. Instead, Oakland instructed the crew to descend and fly westward toward the ocean, where radar could locate them. West of Hamilton Field, there is no ocean. There are the wooded ridges of the Marin hills. The B-17 dropped to 1,500 feet and flew straight into White Hill. The impact tore through the fuselage but did not destroy the aircraft entirely. Lieutenant Skaggs and his co-pilot, both badly injured, managed to pull themselves from the wreckage and stumble downhill through the dark to a convalescent home, where they reached a telephone and called for help.

Civilian rescue teams reached the crash site within an hour, pulling survivors from the crumpled fuselage and recovering the bodies of the two men who had died. Then the military arrived. Military police cordoned off the area and kept onlookers back at gunpoint while cranes were brought in to extract two large crates from the wreckage. The Army denied, firmly and repeatedly, that the B-17 had any connection to the Operation Crossroads nuclear tests. But the plane had departed from the Crossroads training base. The military's response to the crash -- armed guards, heavy equipment, an urgency that went well beyond recovering personal effects -- struck witnesses as disproportionate for an ordinary transport flight. Several crew members added to the questions with their own accounts of the cargo, though nothing was ever officially confirmed. Skeptics have pointed out that the B-17 was already an obsolete aircraft by 1946, and that the military had far more capable planes available for transporting sensitive materials, including the B-29 and C-54.

The official investigation attributed the crash to fuel exhaustion and pilot error -- a conclusion that has puzzled aviation historians given that the plane had refueled just 400 miles earlier and had the range for a transcontinental flight. Whether the fuel gauges malfunctioned, whether the crew made repeated course corrections that burned through reserves, or whether the report simply needed tidy causes for a messy night remains unclear. Today, White Hill is a quiet place. The town of Fairfax lies in the valley below, a small Marin County community of bookstores and coffee shops that feels a long way from nuclear tests and Cold War secrecy. Hikers who climb the ridge can find the crash site, though the forest has long since reclaimed the scar. The wreckage was removed decades ago. What remains is the landscape itself -- the fog that still rolls through these hills on spring nights, the same fog that convinced a crew they were somewhere they were not and sent them descending into terrain they never saw coming.