The USAF countered the MiG-15 by sending over three squadrons of its most capable fighter, the F-86 Sabre. These arrived in December 1950. The MiG was designed as a bomber interceptor. It had a very high service ceiling—50,000 feet (15,000 m) and carried very heavy weaponry: one 37 mm cannon and two 23 mm cannons. They were fast enough to dive past the fighter escort of P-80 Shooting Stars and F9F Panthers and could reach and destroy the U.S. heavy bombers. B-29 losses could not be avoided, and the Air Force was forced to switch from a daylight bombing campaign to the necessarily less accurate nighttime bombing of targets. The MiGs were countered by the F-86 Sabres. They had a ceiling of 42,000 feet (13,000 m) and were armed with six .50 caliber (12.7 mm) machine guns, which were range adjusted by radar gunsights. If coming in at higher altitude the advantage of engaging or not went to the MiG. Once in a level flight dogfight, both swept-wing designs attained comparable maximum speeds of around 660 mph (1,100 km/h). The MiG climbed faster, but the Sabre turned and dived better.