Lockheed SR-71 Blackbird were developed in great secrecy. The U-2 at its time was expected to be invulnerable to defensive measures due to its extreme altitude capabilities. It therefore came as a shock when the Soviets downed one piloted by Gary Powers with a surface-to-air missile. Air combat was also transformed through increased use of air-to-air guided missiles with increased sophistication in guidance and increased range. In the 1970s and 1980s it became clear that speed and altitude was not enough to protect a bomber against air defences. The emphasis shifted therefore to maneuverable attack aircraft that could fly 'under the radar', at altitudes of a few hundred feet. Korean War The Korean War was best remembered for jet combat, but was one of the last major wars where propeller-powered fighters such as the North American P-51 Mustang, Vought F4U Corsair and aircraft carrier-based Hawker Sea Fury and Supermarine Seafire were used. Turbojet fighter aircraft such as Lockheed F-80 Shooting Stars, Republic F-84 Thunderjets and Grumman F9F Panthers came to dominate the skies, overwhelming North Korea's propeller-driven Yakovlev Yak-9s and Lavochkin La-9s. From 1950, North Koreans flew the Soviet-made MiG-15 jet fighters which introduced the near-sonic speeds of swept wings to air combat. Though an open secret during the war, the most formidable pilots today now admit that they were experienced Soviet Air Force pilots, a casus belli deliberately overlooked by the UN allied forces who suspected the use of Russians but were reluctant to engage in open war with the Soviet Union and the People's Republic of China. At first, UN jet fighters, which also included Royal Australian Air Force Gloster Meteors, had some success, but straight winged jets were soon outclassed in daylight by the superior speed of the MiGs. At night, however, radar-equipped Marine Corps Douglas F3D Skynight night fighters claimed five MiG kills with no losses of their own, and no B-29s under their escort were lost to enemy fighters. In December 1950, the U.S. Air Force rushed in their own swept-wing fighter, the North American F-86 Sabre. The MiG could fly higher, 50,000 vs. , offering a distinct advantage at the start of combat. In level flight, their maximum speeds were comparable at about . The MiG could climb better, while the Sabre could dive better with an all-flying tailplane. For weapons, the MiG carried two 23 mm and one 37 mm cannon, compared to