The Spitfire was a low-wing monoplane that was first flown in 1936 and was first put into service with the Royal Air Force in 1938. It was modified continuously throughout the war to serve in a variety of roles: fighter (with notable success at high altitudes), fighter-bomber, and photoreconnaissance plane. The version that entered active service in 1938 had a top speed of about 360 miles (580 km) per hour and an armament of eight .303-inch machine guns. The Spitfire XIV, one of the last models of the war, had a ceiling of 40,000 feet (12,200 m) and a top speed of 440 miles (710 km) per hour; that version shot down more than 300 German V-1 missiles in 1944. During the war the Spitfire's armament was increased to two 20-millimetre cannons along with two .50-inch machine guns or four .303-inch machine guns. Some Spitfire versions could also carry a 250- or 500-pound (115- or 230-kilogram) bomb under the fuselage and a 250-pound bomb under each wing. The last Spitfires in active service (as photoreconnaissance planes) with the Royal Air Force were retired in 1954.