B♭ major or B-flat major is a major scale starting on B-flat. Its key signature has two flats.
Its relative minor is G minor, and its parallel minor is B♭ minor.
B-flat major is a good key for most wind instruments, especially those for which it is their home key, such as clarinets, trumpets, saxophones, and the flutes in B-flat. Because of this, many works for concert bands (those you might have played in while you were at school) are written in this or a closely related key such as F major or E-flat major.
Haydn's Symphony No. 98, which had both trumpet and timpani, is known as the first symphony that anyone had written in that key. Actually, his brother Michael Haydn had written one earlier. However, Joseph Haydn still gets credit for writing the timpani part at actual pitch with an F major key signature (instead of transposing with a C major key signature), something that made things easier and made more sense.
Five of Mozart's piano concerti are in B-flat major.