How "United 93" Came From "Bourne" and "Watchmen"
According to Variety, "The decision to make the $15 million film stemmed from a series of fortuitous events: The demise of a project at Paramount and Universal's desire to make a sequel to "The Bourne Supremacy," which Greengrass had directed.
The Brit director, who had his breakthrough in 2002 with his docudocu-style "Bloody Sunday," had been mulling a film about 9/11 since the attacks in 2001. But in June 2005, the two projects on his plate were "The Bourne Ultimatum" for Universal and an adaptation of "The Watchmen" for Paramount.
Almost since "The Bourne Supremacy" opened in 2004, U had been trying to lock Greengrass down for the third installment. But with no script, neither Greengrass nor star Matt Damon had committed to the pic. So "The Watchmen" at Paramount looked like Greengrass' next film, until newly installed chairman Brad Grey scrubbed the project because of a budget threatening to top $100 million.
With Greengrass available, U execs began trying to get the next "Bourne" entry up and running. Greengrass, however, had another idea: "United 93."
For a whole lot more on the "United 93" back-story, check out this rather fine Variety article.