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 destined for US air units. When new aircraft built overseas did become available, they would be shipped long distances in wartime conditions, with consequent delays and losses. While United States Army Air Forces (USAAF) fighters – such as the Curtiss P-40 Kittyhawk and Bell P-39 Airacobra – damaged during service in Australia could be rebuilt by Australian workshops and loaned to RAAF units, they were not available in sufficient numbers either.
CAC 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 NA-16. While the Beaufort was not a suitable basis for a single-engine fighter, its 1,200 horsepower (890 kW) Pratt & Whitney R-1830 Twin Wasp engines were made under license at the CAC plant in Lidcombe, Sydney and also powered the Grumman F4F Wildcat fighters used by the United States Navy. Consequently, the Twin Wasp was a logical choice for a stop-gap fighter design. The NA-16 had already become the basis of theNorth American NA-50 fighter which had been used by the Peruvian Air Force in the 1941 Ecuadoran–Peruvian War. The Wirraway likewise provided a starting point for the Boomerang’s design.
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, an Austrian Jew who had recently arrived in Australia as a refugee. As David was technically an enemy alien, he wasinterned 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”) (used by the Imperial Japanese Naval Air Service) and the Heinkel He 112 (a contemporary of the Messerschmitt Bf 109 and used in small numbers by Axis air forces in Europe). 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 commenced test flights on 29 May, with pilots Ken Frewin (CAC) and John Harper (RAAF). On 15 July, No. 1 Aircraft Depot RAAF received A46-1 (bu. no. 824) from CAC. Comparison flight tests were undertaken by 1 AD, against a Brewster Buffalo (A51-6) that had been lightened and re-weighted to approximate the flight characteristics of a Zero, as well as a P-40E (A29-129) and a P-400 (BW127). It was found that the Boomerang was faster in level flight than the “Zero”, although the Buffalo out-manoeuvred it. The Boomerang was superior in armament, with two 20 mm cannon and four .303 calibre (7.7 mm) machine guns, all mounted in the short, thick wings. Its pilots were better protected, with generous armour plating, than Japanese fighter pilots. While the CA-12 was lively at low level, its performance fell away rapidly above altitudes of 15,000 ft (4,600 m), and its maximum speed of 265 knots (490 km/h) was not sufficient to make it an effective counter to Japanese fighters like the Zero and theImperial Japanese Army Air Force’s Nakajima Ki 43 (“Oscar”). Similarly, 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.
As test and trial flights commenced, CAC had already begun work on a new variant, the CA-14, to address the Boomerang’s deficiencies in speed, climb and ceiling. 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 it 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 intelligence reports from Europe regarding captured German BMW 801 twin-row radial engines, which were used by the Focke-Wulf Fw 190A) 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 that was 25–30% better than the CA-12, and an operational ceiling 4,000 ft (1,200 m) higher.
Testing of later Boomerang variants found that they 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.