In January 1943, Allied aircraft in New Guinea flew 416 sorties against a single Japanese convoy and sank two ships.

Six weeks later, over three days in the Bismarck Sea, the same air force attacked another convoy and sank all eight transports and half its destroyer escort. Of 6,900 Japanese troops on board, about 1,200 reached their destination. The Allies lost thirteen aircrew.

Nothing new had been invented in between. What changed was how the aircraft were flown, and how low.

The problem: bombs and ships do not get along

Hitting a moving ship with a bomb dropped from altitude is close to impossible, and the Allies had spent a year proving it. The bomb takes half a minute to fall, the ship is doing eight knots and can turn, and the bombardier is aiming at a target the width of a tennis court from four miles up.

In the South West Pacific there was no alternative mission. Japan's factories were far out of range, so the entire job of the Allied bomber force was cutting the sea lanes that supplied Japanese garrisons. The January result, 416 sorties for two ships, was an unmistakable verdict on how that was going.

Group Captain Bill Garing, an RAAF officer on General George Kenney's staff who had flown air-sea operations in Europe, argued for something different: hit the convoy from several altitudes and several directions simultaneously, so that the ships could not manoeuvre away from one attack without exposing themselves to another.

Skip bombing and mast-height bombing

From February 1942 the RAAF had been experimenting with skip bombing, a technique the British and Germans had already tried. The bomber runs in a few dozen feet above the sea and releases; the bomb strikes the water at a shallow angle and ricochets, like a stone skimmed across a pond, into the side of the ship.

The variant that actually won this battle was mast-height bombing, and the numbers are worth stating because they explain the nerve involved. The bomber approaches at 200 to 500 feet at around 265 to 275 miles per hour, then descends to mast height, ten to fifteen feet above the water, roughly 600 yards from the target. It releases at about 300 yards, aiming not at the deck but at the side of the hull.

At that height, the bomb goes into the ship rather than onto it, which is where ships are thin. It also means the aircraft flies directly over the target immediately afterwards, so standard impact fuses would have destroyed the bomber with its own weapon. Crews fitted delayed-action fuses instead. They practised against the wreck of the SS Pruth, a liner that had run aground on a reef off Port Moresby in 1923 and had been sitting there ever since.

None of this works if the ship's anti-aircraft guns are still firing, because a bomber at fifteen feet and 270 miles per hour is a very large, very predictable target. So somebody had to silence the guns first, which required an aircraft that could strafe with real weight of fire.

Pappy Gunn rebuilds the bombers

Major Paul I. “Pappy” Gunn and the 81st Depot Repair Squadron at Townsville in Queensland took Douglas A-20 Havoc light bombers and, in September 1942, packed four .50-calibre machine guns into their noses, adding two 450-gallon fuel tanks for range.

In December they tried the same thing on the larger B-25 Mitchell, and it fought back. The converted aircraft came out nose-heavy even with lead ballast added in the tail. Firing the guns shook the airframe hard enough to pop rivets out of the skin. The tail guns and belly turret came off. What emerged was not a medium bomber any more but a strafer, an aircraft designed to arrive at mast height with a wall of fire ahead of it.

Three days in March

The convoy left Rabaul under Rear Admiral Masatomi Kimura, carrying reinforcements for Lae. Two tropical storms hid it for a while. At about 15:00 on 1 March a patrolling B-24 Liberator found it; eight B-17s were sent and failed to locate the ships.

On 2 March the B-17s attacked from 5,000 feet with 1,000-pound bombs and sank the transport Kyokusei Maru. On 3 March eight Beauforts of No. 100 Squadron RAAF went out with torpedoes; only two found the convoy, and neither scored. Then the weather cleared, and ninety aircraft came off Port Moresby at once.

Thirteen B-17s bombed from 7,000 feet. They hit little, but the ships scattered to avoid them, which broke up the convoy's concentrated anti-aircraft fire and pointed every gun upwards. That was the entire purpose of the high-altitude attack.

It also cost lives. A B-17 broke up in the air, and Japanese fighter pilots machine-gunned the crew as they came down on their parachutes and again in the water. Three P-38s shot down five of those fighters and were themselves lost doing it.

Then came the part that decided it. Thirteen Bristol Beaufighters of No. 30 Squadron RAAF ran in at wave-top height in line abreast, deliberately looking like Beauforts making a torpedo attack. The Japanese captains did the correct thing against torpedoes and turned their ships bow-on, presenting the narrowest possible target.

Bow-on is the worst possible aspect against strafers. The Beaufighters flew straight down the length of each ship with four 20 mm cannon in the nose and six .303 machine guns in the wings, and swept the bridges, the gun positions and the crews. Aboard one of them was the cameraman Damien Parer, whose footage became the newsreel The Bismarck Convoy Smashed.

Immediately behind them, seven B-25s bombed from about 2,500 feet while six more came in at mast height. The convoy had been disorganised from above, blinded at deck level, and was now being attacked from ten feet by aircraft carrying delayed-fuse bombs aimed at its waterline.

Then MacArthur got hold of it

The real result was crushing enough: eight transports and four destroyers sunk, about 2,890 Japanese dead, the reinforcement of Lae by sea effectively ended.

The communiqué General Douglas MacArthur issued on 7 March claimed 22 ships sunk, including twelve transports, three cruisers and seven destroyers, and 12,792 Japanese troops killed. Army Air Force headquarters in Washington examined the claim in mid-1943 and concluded that only sixteen ships had been involved in the first place, so twenty-two of them could not have been sunk. General Headquarters in the South West Pacific declined to correct the record.

He was right about the significance and wrong about the arithmetic, which is a fairly typical combination. Inflated claims were endemic on every side in 1943; gun-camera film was scarce, smoke obscured everything, and a dozen aircrew watching the same explosion honestly reported a dozen sinkings.

The tactical lesson survived the exaggeration intact. Land-based aircraft, flown low enough and in sufficient coordination, could annihilate a defended convoy in daylight without a single warship present. Every navy in the world read that report.

Japan did not attempt another large troop convoy to Lae. Reinforcement continued by submarine, by barge at night, and on foot. Which is another way of saying that four days of flying at fifteen feet closed a shipping lane for the rest of the war.

Sources: Australian War Memorial; U.S. Army Air Forces official histories; Air & Space Forces Magazine; Anzac Portal, Department of Veterans’ Affairs; British Pathé and Australian newsreel archives.