Here is a quote from an article titled "Nelly Producer Gives Story Behind 'Tilt Ya Head Back'"
Nelly's "Tilt Ya Head Back" with Aguilera sounds tailor-made for the two, but Doe says it was meant as a song for Nelly and Britney Spears. He says Spears' label pulled out because the song sounded too urban. Aguilera and Janet Jackson were considered next, and Aguilera landed the song, which coasts on a heavy "Superfly" sample.
Source: St. Louis Post-Dispatch