Station HYPO

To boost morale following the defeat at Guadalcanal (November 12-15, 1942), Admiral Isoroku Yamamoto, commander of the Imperial Japanese Navy’s Combined Fleet decided to make an inspection tour throughout the South Pacific during the month of April, 1943.

However, the Shootdown of Admiral Yamamoto on April 18 was directly connected to a battle at sea that occurred three months earlier between New Zealand and Japan.

During the night of January 29, 1943, while the Japanese submarine (l-1) was submerged near Guadalcanal, the two New Zealand corvettes HMNZS Kiwi and her sister ship the HMNZS Moa picked up her phosphorescent.  The Kiwi launched six depth charges and followed with six more, forcing the submarine to surface.  The I1 ascended and tried to boat off while exchanging gun fire with both ships.  The Kiwi rammed the I1 damaging itself and the I1.  The Moa took over the fight and chased the submarine further towards shore while firing its 4” gun.  Two hours after the initial attack, the I1 unable to escape stranded in shallow waters on a reef.  Because of the JN-25 code books the I1 carried, the Japanese Sailors on board tried to blow up the submarine while abandoning it.  Fortunately, the Japanese failed total destruction!

The Intelligence

Immediately after the sinking, the Allies started to explore the wrecked I1 and recovered about 200.000 pages of the JN-25 code books, charts and manuals. The code books included updated Japanese Navy codes which were immediately send to Station Hypo at Pearl Harbor where the code breakers used them to exploit the Imperial Japanese Navy communication messages.

On April 14, 1943, a couple of days before the Admiral’s schedule tour, the US naval intelligence intercepted and decrypted a message containing specific details regarding Yamamoto’s tour. 

The decrypted message provided time-sensitive information such as times of arrival and departures, the number and types of planes that would transport and accompany the Admiral on the journey as well as the flight route from Rabaul to Ballalae Airfield on an island near Bougainville in the Solomon Islands on the morning of April 18, 1943.

This tactical time-sensitive information immediately went from Commander in Chief Pacific Fleet Adm. Chester W. Nimitz to Secretary of the Navy Frank Knox who delivered the news to President Franklin Roosevelt. Reportedly, the president’s response was, “Get Yamamoto.” Regardless of whether or not the president actually said those words, the order was given: kill the mastermind of the Pearl Harbor raid.

The plan

Although the Navy initiated the plan, no Navy or Marine fighter aircraft could fly the mission. Their range was inadequate for the 1,000-mile round trip and an aircraft carrier could not approach a Japanese-held island without being detected. Instead, the 339th fighter squadron was assigned the task.

Maj. John Mitchell, commanding officer of the 339th, chose 17 of his most reliable Lightning pilots and organized them into nine divisions of two aircraft each. The task of shooting down Bettys went to the four-plane Killer Division. Lt. Rex Barber was assigned to this division as the wingman for Capt. Thomas Lanphier.

The punctual Yamamoto was expected to arrive on Ballale at a precise time the morning of the 18th. Mitchell calculated a flight plan for his own Lightnings to intercept the admiral’s plane at a point over Bougainville Island. For increased accuracy in navigation, Mitchell installed a navy ship’s compass in his Lightning. Because of the great distance, larger wing tanks were fitted to the P-38s. Barber later said that they “didn’t know if they’d get off the runway” carrying the extra weight.

Because of the many unknown variables, Mitchell estimated the likelihood of the plan’s success at 1,000-1.

The mission

Takeoff from Henderson Field was at 0630. Barber flew another pilot’s P-38G with “Miss Virginia” painted on its nose. His own Lightning, “Diablo,” was down for repairs. All the fliers got airborne except one member of the Killer Division who blew a tire on takeoff.

Another turned back after he found his fuel tanks were not feeding properly. The vacancies were filled by backups Ray Hine and Besby Holmes.

The group flew directly west from Guadalcanal and over the Solomon Sea. They flew just over the horizon to avoid detection.

Navigating the five legs of the flight with no visual references was tricky. Mitchell used only his airspeed indicator, compass and timepiece. The other pilots were ordered to follow behind and turn with him. Strict radio silence was observed.

The flight flew 50-100 feet above the waves. Flying that low in the tropics, heat, humidity and boredom became factors. Mitchell himself admitted to sleepiness. When another pilot nodded off, the tips of his props clipped a wave top, spraying seawater over his canopy that dried into a haze. Pilot Doug Canning kept alert by counting sharks; he spotted 48.

The Kill

After two hours, Mitchell was convinced they had missed their target. On the final leg, a northeast turn toward Bougainville, he spied what he thought was a beach. At that moment Doug Canning (with 20-10 vision) shouted, “Bogeys 10 o’clock high!”

Sure enough, nearly straight ahead were at least two Japanese Betty bombers on a southwesterly course. The chase was on.

The cover pilots immediately accelerated and climbed toward the spot above the bombers where escorting Zeros were expected to be lurking. The Killer Division headed straight in. Holmes could not drop his external tanks and pulled aside with Hine. This left Barber and Lanphier to address the matter of the Bettys.

Lanphier turned up and away to confront two Zero fighters that were diving down. Barber was alone. His account of what then happened is this:

He spotted at least two Betty bombers and approached them from 90 degrees on their right. When Lanphier peeled up, he slid in behind one Betty. He lost sight of the other (which he later learned was under him). From this ideal position astern his prey, he opened fire on the bomber with his four .50-caliber machine guns and 20-mm cannon. He swung his Lightning back and forth, left to right, and raked the entire Japanese aircraft with bullets. He saw pieces of the plane’s rudder and engine cowling fly off.

The engine began smoking badly as the plane snapped left, its right wing narrowly missing Miss Virginia. The Betty then plunged into the jungle of Bougainville. Barber saw no other P-38s participate in the attack. He noted a column of black smoke coming up from the jungle.

The pursuing Zeros caught up with him and managed to inflict 52 (104 in and out) hits on Miss Virginia including seven through the propeller blades. Barber sped low over the jungle treetops and out to sea where he encountered another struggling Betty, which he also shot down. He had now done all that he could so, low on fuel, it was time to get back to Henderson.

Sources:
NSA.gov
warhistoryonline.com
Source: cryptologicfoundation.org