The B720 was a smaller version of the B707, It was designed to be used at smaller airports with smaller runways and pax loads. I am not at all sure but I think that UA was the first operator. I know that Pan Am operated them as well; although briefly. The B720 was quickly dumped after the 727 arrived. It was called B720 because UA did not want customers to here 707-X as (Boeing was planning to call it) and think that they were boarding an inferior type of B707.