In April 1943, Zeamer and the crew had been approached about a solo, 1,200-mile (1,900 km) round-trip photo-mapping mission of the western coast of Bougainville, with emphasis on Empress Augusta Bay where any marine landings would be made. Such maps were considered vital to a future invasion of the island in support of coalescing plans for the reduction of Rabaul. It was presented as a volunteer mission because extended mapping runs would require straight and level flight runs of up to 22 minutes deep in hostile territory.
The necessary weather for such a run proved elusive for two months, until mid-June. When the 8th PRS was unable to get the necessary photos on June 15, Zeamer was contacted again. At 4 a.m. the next morning, 16 June 1943, after intense preparations the day before, the crew headed for Bougainville. Twice already, once the night before and once as the aircraft was taxiing for take-off, they were ordered by V Bomber Command to do a photo recon of the Japanese airstrip on Buka, a small island off the northern tip of Bougainville. Zeamer rejected the idea both times as too dangerous, almost guaranteeing interception by enemy fighters while in sustained level flight for the mapping.
Arriving too early at Bougainville to start the mapping, Zeamer put the question of whether to pass the time over the ocean or go the Buka recon to the crew. Voting for the recon, Zeamer flew northeast in a loop to come back over Buka on their way into the mapping run. Photos taken that day reportedly showed twenty-one Japanese fighters taxiing or taking off to intercept. With approximately a minute left in the mapping run, "Old 666" faced a coordinated attack by eight A6M3 Model 22 Zero fighters from 251 Kōkūtai, as well as an unidentified twin-engined fighter. The ensuing attack mortally wounded bombardier Sarnoski, who struggled back to his gun to drive off a second Zero after being blown from his position by a 20 mm cannon shell from the first. Another of a total of four 20 mm shells destroyed the pilot's side of the instrument panel and broke Zeamer's left leg above and below the knee and leaving a large hole in his left thigh. He was also hit by shrapnel in both arms and his right leg, with a gash in his right wrist. Three others were also wounded, including the navigator and top turret gunner, who responded to a resulting oxygen fire by putting it out with their bare hands.
Due to the loss of oxygen and to escape their attackers, Zeamer dived the plane violently from 25,000 feet (7,600 m) to approximately 10,000 feet (3,000 m), estimating the altitude by an increase in engine manifold pressure. The Japanese followed them down and commenced a forty-minute series of passes at the nose of the B-17. Despite his wounds, Zeamer avoided any further extensive damage to the B-17 by repeatedly turning into the oncoming fighters just inside the trajectory of their fixed fire, a technique he learned while in the 22nd BG. By doing so, the Zeros would continue rolling into the Fortress without hitting it, but exposing themselves to the rear guns of the B-17. Eventually, all of the Zeros broke off due either to damage, lack of ammunition, or lack of fuel.
After the engagement, an assessment revealed that the B-17's oxygen and hydraulic systems were destroyed, as well as all of the pilot's flight instruments. The magnetic compass and engine instruments on the copilot's side were undamaged, as were all four engines. Too wounded to move and unwilling to give up command of the plane, Zeamer advised the top turret gunner as he took over copiloting duties, allowing the unwounded copilot to attend to the wounded. The lack of oxygen, in addition to Zeamer's and the navigator's injuries, meant a return to Port Moresby over the Owen Stanley Mountains was impossible. Instead, they made an emergency landing at an Allied fighter airstrip at Dobodura, New Guinea. Without operable brakes or flaps because of the destroyed hydraulic system, the B-17 was ground-looped without incident by the co-pilot. The casualties were one killed (Sarnoski) and four wounded. Zeamer was initially thought dead from loss of blood, but was treated with the other injured crew members by the 10th Field Ambulance of the Royal Australian Army Medical Corps before being transported back to Port Moresby the next day.
Colonel Merian C. Cooper, chief of staff to the deputy commander of the Fifth Air Force, Major General Ennis Whitehead, recommended Zeamer be awarded the Medal of Honor, to which Fifth Air Force commander General George Kenney concurred. Zeamer received the award from Chief of the Army Air Forces General Henry H. Arnold on January 16, 1944, at the Pentagon.
Sarnoski was also awarded the Medal of Honor, marking only the third time in U.S. history that two members of the same crew received the Medal of Honor for a single mission. (The others were Robert G. Robinson and Ralph Talbot in World War I, and Addison Baker and John L. Jerstad just two months after Zeamer's Medal of Honor action during the Allied raids on oil refineries in Ploieşti, Romania.) All other members of Zeamer's crew received the Distinguished Service Cross. It remains the most highly decorated single air mission, and Zeamer's regular crew the most highly decorated, in American history.