During WW2 they forbade the use of proximity shells over land until late in 1944 out of fear the Axis could reverse engineer it. The same worry now surrounds the F-35. There was a similar concern round the Norden bombsight.
“We recovered the wreckage and determined it was from the F-35,” a spokesman for the Air Self Defense Force (ASDF) said, adding that the pilot of the aircraft was still missing.
Reuters
The advanced, single-seat jet was flying about 135 km (84 miles) east of the Misawa air base in Aomori Prefecture at about 7:27 p.m. (1027 GMT) on Tuesday, when it disappeared from radar, the Air Self Defense Force said.
The aircraft was less than a year old and was delivered to the ASDF in May last year, the spokesman said. Japan’s first squadron of F-35s has just become operational at Misawa and the government plans to buy 87 of the stealth fighters to modernize its air defenses as China’s military power grows.
In WW2 the captain of the Sculpin, who knew the Navy had broken the Japanese codes, deliberately went down with his ship rather than risk the secret.
This action left CAPT Cromwell facing a fateful choice. With his personal knowledge of both ULTRA and GALVANIC, he realized immediately that to abandon ship and become a prisoner of the Japanese would create a serious danger of compromising these vital secrets to the enemy under the influence of drugs or torture. For this reason, he refused to leave the stricken submarine and gave his life to avoid capture. He and 11 others[7] rode Sculpin on her final plunge to the bottom, where her secrets would be safe forever.
Wikipedia
The Army Air Corps rescued downed airmen and their Norden bombsights by dogsled from Greenland to keep them secure.
For a moment it looked as if he’d made it, but then the nose wheel collapsed and the airplane flipped over. The remaining P-38s all landed with their wheels retracted. The B-17s stayed up for another hour or so, sending out S.O.S. signals before they too bellied onto the harsh and desolate site.
Air and Space Magazine
For nine days, the 25 men on the flight huddled inside the two B-17s, where they lived, all things considered, in relative comfort. There was little concern about rescue—supplies had been dropped on the third day, and word came that a rescue team was on its way. Men from a special Army Air Forces unit driving a dogsled finally arrive on July 24 to lead the downed crew on an arduous 10-mile march to the south-east coast of Greenland, where a Coast Guard cutter would be waiting.