The Large Area Telescope on board the Fermi satellite observed a gamma-ray flare in the Crab nebula lasting for approximately nine days in April of 2011. The source, which at optical wavelengths has a size of ~11 ly across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 +- 6) 10-7 cm-2 s-1 above 100 MeV, which corresponds to a 30-fold increase compared to the average value. During the flare, a new component emerged in the spectral energy distribution, which peaked at an energy of (375 +- 26) MeV at flare maximum. The observations imply that the emission region was likely relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability. Show Partial Abstract