Was the Boeing 747 designed to be supersonic?
No. The Boeing 747 was not designed for supersonic flight, though during testing, it was pushed quite close to the sonic speed:

Tom Cole, a spokesman at Boeing Commercial Airplane Co., said original flight tests of 747s conducted in 1969 and 1970 took 747-100 models to speeds of Mach 0.99.

However, the aircraft is not built for sustained supersonic speeds:

... Boeing and the FAA said the 747 is not built for sustained flight at the speed of sound. Its engines aren't powerful enough nor is it designed to deal with a destabilizing shock wave that develops around the speed of sound.

However, it is just possible that the airframe and wings can cope up with the supersonic flight, at least for a little while, though any sustained flight is not possible. There have been cases where the 747 has gone quite near the speed of sound and lived to tell the tale.
China Airlines Flight 006 went into a dive and went near supersonic speeds before the captain could recover it. As a result, large portions of the horizontal stabilizers were ripped off, and the wing was bent permanently upwards.

Damage to China Airlines Flight 006 empennage. By NTSB - China Airlines Boeing 747-SP Accident Report by NTSB, http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/ChinaAir/AAR8603.html, Public Domain, Link
In 1991, a Boeing 747 of Evergreen airlines entered a steep right wing bank and approached sonic speed before recovery was completed. NTSB notes:

... the airplane was in a steep right-wing-down bank. The flight  lost  approximately 10,000 feet of altitude, and the airplane approached supersonic speeds (0.98 Mach) before the recovery could  be  completed.

Later, the Israelis took it close to sonic speed. In these cases, parts of the aircraft certainly experienced local supersonic flow; however, the airframe and wings are not designed for supersonic flow nor are the engines powerful enough. In one case, the aircraft suffered extensive damage (though it was repaired and returned to service).