‘40’ was the last track recorded for the “War” sessions. Bono called the song “40” as he took the lyrics from Psalm 40, written by King David, in the Bible.

 U2 had booked the studio at Windmill Lane Studios in Dublin for so many hours and at the end they had time remaining. Adam Clayton, the group's bassist, had stepped out and left the studio. Bono, the Edge and Larry Mullen Jr. decided they didn't have a good song to end the album. They came up with the concept and the song was recorded quickly with the Edge switching off and playing both the electric and bass guitar parts. 

It was a staple of U2 shows for years and was used to close out the night with the audience often singing the last verse long after the band had left the stage, even as they exited to the parking lots.