The CAC Boomerang was a World War II fighter aircraft designed and manufactured in Australia between 1942 and 1945. The Commonwealth Aircraft Corporation produced Boomerangs under the production contract numbers CA-12, CA-13, CA-14 and CA-19, with aircraft supplied under each subsequent contract incorporating modifications and improvements. The Boomerang is significant as the first combat aircraft designed and built in Australia.
The Pacific War began on 7 December 1941 with surprise attacks by the Empire of Japan on Pearl Harbor, Thailand, Malaya, and the Philippines. Within a few months, Japanese forces had conquered vast areas of the Pacific and South East Asia. During these campaigns, the ill-prepared Allied air forces in the Pacific suffered devastating losses.
Because of political and cultural ties between the United Kingdom and Australia, British manufacturers were the main source of RAAF aircraft. However, the British aircraft industry had long been hard-pressed to meet the needs of the RAF. Although United States companies had enormous aircraft manufacturing capacity, their output was now intended first and foremost for U.S. air units. Even if aircraft built overseas did become available, they would be shipped long distances in wartime conditions, with consequent delays and losses. CAC came into its own: work re-commenced on an existing bomber design, the Woomera, and CAC also examined the possibility of designing and building fighters.
The main challenge was the fact that fighter aircraft had never been built in Australia. Only two military aircraft were in production at the time: the Bristol Beaufort twin-engined bomber and the CAC Wirraway, a single-engine armed trainer/ground attack aircraft, based on the North American Harvard. While the Beaufort was not a suitable basis for a single-engine fighter, its 1,200 horsepower (890 kW) Pratt & Whitney Twin Wasp engines were made under license at the CAC plant in Lidcombe, Sydney and also powered the Grumman Wildcat fighters used by the U.S. Navy. Consequently, the Twin Wasp was a logical choice for a stop-gap fighter design. The Harvard had already become the basis of the North American NA-50 (P-64) fighter (which had already seen action with the Peruvian Air Force). The Wirraway likewise provided a starting point for the Boomerang's airframe.
Like the latest fighters at the time, planning for the Boomerang included automatic cannons. As no such weapons were manufactured locally, a British-made Hispano-Suiza 20 mm which an Australian airman had collected as a souvenir in the Middle East was reverse engineered.
Lawrence Wackett, general manager and former chief designer of CAC, recruited designer Fred David, a recently-arrived Austrian Jewish refugee. As he was technically an enemy alien, David had been interned by Australian immigration officials. He was well-suited to the CAC project, since he had previously worked for Heinkel in pre-Nazi Germany, as well as Mitsubishi and Aichi in Japan. As a result, David had an excellent understanding of advanced fighter designs, including the Mitsubishi A6M ("Zero") and the Heinkel He 112. Design work began on 21 December 1941, at the CAC factory in Fishermans Bend, Melbourne.
The Boomerang was a small fighter, designed with an emphasis on manoeuvrability. It had an overall length of just 7.7 metres (25.5 ft) and an 11 m (36 ft) wingspan. Although the original intention had been to use as many Wirraway components as possible, the final design was quite different, with shorter wings, a shorter, wood-sheathed, aluminium-framed fuselage, increased strength for combat stresses and a new centre section.
On 2 February 1942, before the debut flight of the Boomerang's prototype, the RAAF ordered 105 CA-12 (Mark I) variants. The prototype took to the air on 29 May.
Test flights found that the CA-12 handled well. It was very well-armed, with two 20 mm cannon and four .303 calibre (7.7 mm) machine guns, all mounted in the short, thick wings. The Boomerang was also generously equipped with armour plating to protect the pilot. However, general performance was mediocre. Although lively at low level, performance fell away rapidly over 15,000 ft (4,600 m), and at the maximum speed of 265 knots (490 km/h) was not sufficient to make it an effective counter to the Zero. In addition, the best European fighters were reaching almost 350 knots (650 km/h), and even relatively sluggish fighters like the Wildcat and the Kittyhawk (which would become the main fighter used by the RAAF during the war) were much faster than the Boomerang
In early 1942, CAC began work to address the CA-12's deficiencies in speed, climb and ceiling. A new variant, the CA-14, was designed around an order for 145 U.S.-built, 1,700 hp (1,268 kW) Wright Cyclone R-2600 engines. However, the Wright engines ordered were not delivered as scheduled, and in mid-1942 Wackett authorised use of the 1,850 hp (1,380 kW) Pratt & Whitney R-2800, which was available from the CAC factory in Lidcombe. However, the significantly greater weight of this powerplant led to an unacceptable risk of undercarriage failure. (The R-2800 engine would later be the basis of design work on the Boomerang's successor, the CAC CA-15, also known as the "Kangaroo".)
CAC eventually returned to the Twin Wasp, to which was added a General Electric B-2 turbo-supercharger mounted inside the rear part of the fuselage, new propellor gear, a geared cooling fan (influenced by reports on Focke-Wulf Fw 190As captured in Europe - a standard feature of its BMW 801 twin-row radial engine) and a larger, squared-off tailfin and rudder. By July 1943, the significantly re-worked CA-14 prototype, now known as the CA-14A, had a top speed which was 25–30% better than the CA-12, and an operational ceiling which was 4,000 ft (1,200 m) higher. Overall, it compared favourably with the Spitfire Mk V and early Thunderbolts and Mustangs. By this time, however, British-built Spitfires had filled the interceptor role and Mustangs had been ordered, to fill the bomber escort, air superiority and close air support roles. Consequently, production Boomerangs were never fitted with superchargers.
Boomerangs that reached frontline RAAF units were delivered under three different CAC production contract numbers: CA-12, CA-13 and CA-19, incorporating various minor improvements and modifications. A total of 250 aircraft of these marques were built: 105 CA-12s, 95 CA-13s and 49 CA-19s  The CA-13 and CA-19 are sometimes known collectively as the Boomerang Mark II.
The sole CA-14A was used for research by No. 1 Aircraft Performance Unit RAAF, and was also seconded to the Bureau of Meteorology for a period after the war ended.
A No. 4 Squadron Boomerang and ground crew at Nadzab, New Guinea in October 1943
CAC Boomerang during assembly
Following the devastating first air raids on Darwin on 19 February 1942, the need for interceptors became more pressing. Despite the Boomerang's astonishingly short development phase — especially since the Australian aviation industry had never built fighters before, let alone designed them — by the time the Boomerang entered service, sufficient Curtiss P-40 Kittyhawks had arrived from the United States. In January 1943, these were replaced in the air defence role over Darwin by No. 1 (Fighter) Wing RAAF, which had returned from Europe, equipped with the Spitfire Mk Vc.
Two of the first three operational Boomerang units, No. 83 Squadron and No. 85 Squadron, were used for home defence. No. 84 Squadron was deployed to Horn Island in Torres Strait and Dutch New Guinea (West Papua), in an attempt to address the continuing shortage of fighters in this area. The squadron was only modestly successful however. The Boomerang's low top speed and poor high altitude performance meant that No. 84 Squadron could drive off enemy attacks but rarely get close enough to Japanese aircraft to bring their guns to bear. On the only occasion when a Boomerang did close with a Japanese aircraft, its guns jammed. Fortunately, there were not many air raids in this area, and after using Boomerangs for eight months No. 84 Squadron upgraded to the Kittyhawk.
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 terms, a 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 (usually the aircraft were based close to the area of ground operations), 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. Sprightly low-level handling helped avoid ground fire and rough terrain and the unusually extensive armour plating protected pilots. In addition, the aircraft's simple wood and aluminium airframe proved capable of absorbing battle damage. Nonetheless, several of the aircraft were shot down, including two accidental kills by U.S. units, and more were damaged during landing accidents, as the Boomerang was prone to ground looping.
No. 4 Squadron and No. 5 Squadron flew Boomerangs in New Guinea, the Solomon Islands Campaign and Borneo Campaign, also in the close support role, with marked success. Flying in pairs (one to observe the ground, the other to observe the air around them), their tasks included bombing, strafing, close infantry support and artillery spotting. When attacking larger enemy formations Boomerangs often operated in conjunction with larger aircraft. In this role the Boomerang would get in close to confirm the identity of the target and mark it with a 20 lb (9 kg) smoke bomb with the "cooperating" aircraft delivering the major ordnance from a safer distance. The partnership between 5 Squadron Boomerangs and Royal New Zealand Air Force Corsair fighter bombers during the Bougainville Campaign was said to be particularly effective.
The other user of Boomerangs in New Guinea was No. 8 Communications Unit, which used the aircraft to assist with air sea rescue operations.