To play the B7 chord, perform a barre by pressing your index finger on the top g-string, E-string, and bottom-A string on the 2nd fret and middle finger on the 3rd fret of the C-string.
B7 Ukulele Chord Explanation
The B7 chord contains the notes B-D#-F#-A.
To build a B7 chord, take the dominant seventh formula (1-3-5-b7) and apply it to the B major scale (B, C#, D#, E, F#, G#, A#).
Identify the first, third, fifth, and flat seventh scale degrees in the B major scale which are B-D#-F#-A.
Notice how the seventh scale degree found in the chord is lowered a half step as indicated by the "flat" or "b" that precedes the number "b7". In this case, the seventh scale degree of an B major scale – an "A#" note is lowered a half step to an "A" note to make the chord a dominant seventh chord.
Watch the following video to learn how to play an B7 chord on ukulele.