SUPERNOVA 1993J in the galaxy M81 is the second-brightest type II supernova observed this century, surpassed only by SN1987A in the Large Magellanic Cloud. Here we report the evolution of the photometric and spectral properties of SN1993J for the first 50 days following its discovery. The behaviour of this supernova is unusual, showing features typical of type II supernovae near the initial maximum, but with the strong helium lines characteristic of type Ib supernovae at later times. This implies that the progenitor star had an unusually thin hydrogen envelope (compared to normal type II progenitors), suggesting that significant mass loss had taken place before the explosion. Application of an expanding photosphere model1 to our data provides an estimate of the distance to the supernova of 2.6 +/- 0.4 Mpc, broadly consistent with the distance to M81 determined using Cepheid variable stars2. Supernova models that more closely match the atypical spectral features of SN1993J may change the inferred distance, and should provide better constraints on the structure of the progenitor.