The CAC Boomerang was a World War II fighter aircraft manufactured in Australia between 1942 and 1945.

Japan entered the war in December 1941 and within just a few months had conquered vast areas of the Pacific and South-east Asia, in the process destroying the great majority of Allied aircraft opposed to them. As early as January 1942, Japanese forces were occupying bases just a few miles to the north of Australia and threatening to invade. For Australia, the situation was desperate: her elite fighting forces were half a world away in Europe or the North African desert helping England to fight Germany and Italy, and there was not a single modern fighter aircraft in the country.

Australia's traditional aircraft supplier, England, was already hard pressed to meet its own needs, and the Churchill government took the attitude that the defence of British colonies was unimportant. The United States was the only other possibility, but the enormous manufacturing resources of that country were only just starting to produce fighter aircraft in quantity, and there was no certainty that any could be spared for Australia.

Within days of the Japanese surprise attacks on Pearl Harbor, Siam, Malaya, and the Philippines—some reports say only three days—the tiny Australian aircraft industry had started designing a fighter with whatever components were available. Only two aircraft were in production in Australia at that time: the Bristol Beaufort torpedo bomber, and the CAC Wirraway advanced trainer. The twin-engined Beaufort was obviously unsuitable as the basis of a fighter, but it did have reasonably powerful engines, 1200 horsepower (890 kW) Pratt & Whitney Twin Wasps, which were already being made under license in Sydney. With its high frontal area it was not the most promising engine for a fighter, but it was the only one available.

For the airframe, the Wirraway trainer could provide a starting point. From the roughed-out plans, Commonwealth Aircraft Corporation general manager and former chief designer Lawrence Wackett and current chief designer Fred David began detailed design work on 21st December 1941. The RAAF ordered 105 Boomerangs on 2nd February 1942, and the prototype flew on 29th May 1942.

It was a very small aircraft by fighter standards, designed for manoeuvrability more than speed, with an overall length of just 7.7 m and an 11 m wingspan. (See the table below. In particular, compare with the rather similar Grumman Wildcat.) Although the original intention had been to use as many Wirraway components as possible, the final design was quite different, with shorter wings and shorter, wood-sheathed, aluminium-framed fuselage, increased strength for combat stresses, and a new centre section.

The Boomerang handled well and unlike most initial models of fighter aircraft, was well-armed from the start with two 20 mm cannon and four .303 machine guns (~7.7 mm), all mounted in the short, thick wings, and it was generously equipped with armour plating to protect the pilot. Outright performance was mediocre, however. Although lively at low level, it fell away rapidly over 15,000 feet (4,600 m) and at 265 knots top speed (491 km/h) was not sufficient to make it an effective counter to the Zero. (In 1942 and '43, the best European fighters were reaching almost 350 knots (650 km/h), and even relatively sluggish aircraft like the Wildcat and the Kittyhawk were comfortably faster.)

Despite the astonishing speed of design and production (particularly so given that Australia was a country that up until then had never made a fighter) by the time the Boomerang entered squadron service the immediate need for fighters had been filled, initially by Curtiss Kittyhawks from the United States (starting in March 1942), and then by the return from Europe of two RAAF squadrons equipped with Spitfire Vs which, together with an RAF Spitfire squadron, arrived in Darwin in January 1943. Of the first three operational Boomerang units, 83 and 85 Sqns were used for home defence but the continuing shortage of fighters saw 84 Sqn deployed to New Guinea for use as interceptors, with only modest success. The Boomerang's low top speed and poor high altitude performance meant that 84 Sqn could drive off enemy attacks but rarely get close enough to Japanese aircraft to bring their guns to bear. It was a measure of the Boomerang's limitations as a fighter that after 8 months in New Guinea 84 Sqn upgraded to the lacklustre Kittyhawk instead.

The Boomerang found its real use as a close support aircraft. In contrast to Europe or North Africa, the ground war in the jungles of the south-west Pacific was, in broad, an endless series of small unit actions fought at very close quarters by widely dispersed forces with no clear front lines. It was here that the Boomerang found its niche: as close to the troops on the ground as possible.

It had the range to go wherever it was needed, heavy armament by the standards of the day, and because it was easier to fly than most fighters, the pilot could get in close to the objective and have time to concentrate on the ground forces, rather than on not flying into the terrain. Sprightly low-level handling helped avoid ground fire, the unusually extensive armour plating protected pilots, and the simple wood and aluminium airframe proved very capable of resisting battle damage.

RAAF 4 and 5 Sqns flew Boomerangs in New Guinea, the Solomons, and Borneo in the close support role with marked success. Tasks included bombing, strafing, close infantry support, and artillery spotting. For larger enemy formations, Boomerangs often operated together with heavier aircraft, the Boomerang getting in close to confirm the identity of a target and mark it with a 20 lb (9 kg) smoke bomb, the heavier craft delivering the major ordinance from a more practical distance. The partnership between 4 and 5 Sqn Boomerangs and RNZAF Corsair fighter-bombers was said to be particularly effective.