By early 1942 the Japanese had assembled a force of nearly 130 aircraft in the islands to the north-west of Darwin. This force, comprising sixty-three Mitsubishi G4M (Betty) bombers, forty-eight Mitsubishi A6M (Zero) fighters, and eighteen four-engined flying boats, had been assembled from elements of the 21st and 23rd Air Flotillas which had already participated in the Philippines campaign and the sinking of the British battleships HMS "Prince of Wales" and HMS "Repulse". From bases in Java, Timor, Celebes, and New Guinea, the Japanese attacked every operational airfield in the northern and north-western parts of Australia, including the strategically important Allied airfields at Townsville and Horn Island in north Queensland. The Allied supply ships that plied the Arafura Sea and Torres Strait were also vulnerable to these attacks.