The String Quintet No. 1 in B-flat major, K. 174, was written by Wolfgang Amadeus Mozart in December 1773. Unlike Mozart's other viola quintets, which are scored for two violins, two violas and cello, this early work is scored for two violins, two violas and basso. It is inspired by Michael Haydn viola quintets in C major (MH 187) and G major (MH 189), written earlier in the same year.
Mozart composed this quintet a few months following the composition of the Viennese Quartets, K. 168–173 – indeed this work immediately follows that set in the first edition of the Köchel catalogue.