Complex sentence: The design is clearly based upon the VF-17 Nightmare (but with design lines that resemble those of the VF-19 and the VF-22), the VF-171 was developed as a multipurpose heavy space fighter.
Split into simpler sentences: The design is clearly based upon the VF-17 Nightmare, but it also has design lines that resemble those of the VF-19 and the VF-22. The VF-171 was originally developed as a multipurpose heavy space fighter.