No. Although the US did fly daylight bombing missions at heights over 25,000 feet, this did not make them immune from interception by manned fighters or from flak. Now it is true that this altitude did pose a number of problems for Axis air defense. Both Japan and, to a lesser extent, Germany lagged in the production of superchargers that would have allowed their fighters to behave at maximum performance at these altitudes. Axis interceptors could reach the bombers, but they lacked the performance they had at lower levels. Japanese interceptors in particular had difficulty making multiple passes against the B-29s. But bombing at high altitudes also degraded bombing accuracy as bombers flew through air currents like the jet stream. The poor results of B-29 daylight attacks was part of the reason why LeMay switched to low altitude fireboming attacks at night. Nonetheless, American bombers could and were intercepted while flying at altitude. Altitude posed a problem for manned fighters, but not an insurmountable one, and was not much of an issue for heavy flak. This USAAF video shows the tactics used to spoof anti-aircraft fire showing the real dangers of antiaircraft fire posed to bombers trusting to fly a single course, even at high altitudes.