"Feelin' Stronger Every Day" was written by Peter Cetera (vocalist, bassist) and James Pankow (trombones, percussion) of Chicago. It was released as a single from their two-time multi-platinum album Chicago VI.
In a 1991 interview, Peter Cetera said that while the song is about "both healing and moving on after the end of a relationship," it could just as well apply to the dynamic wave of success that Chicago was riding at this time. This was their second #1 album on the US album charts, their third two-time platinum album, and they would yet have two more consecutive #1's on the US album charts. This song comes right at what many could argue was the peak of their career, although even the lowest of Chicago's career could pass for moderate success for other bands.
James Pankow just happens to have composed both of the album's single hits; the other one is "Just You 'N' Me." Robert Lamm (keyboardist, vocals) usually was the group songwriter.
A live version also appears on 1999's Chicago XXVI - The Live Album.
1973 was a busy year for Chicago - their manager, James William Guercio, produced and directed the film Electra Glide in Blue, which had many Chicago members in minor roles as well as featuring Chicago on the soundtrack. The film, being a motorcycle flick shot in Monument Valley, Arizona, has had some small cult following. In order to ensure the budget could afford a good cinematographer, Guercio accepted a salary of just one dollar for directing.