Complex sentence: The decision was based not only on aircraft performance but also on contractor performance; during the U- 2 project Lockheed had proven its ability to design advanced aircraft in secret, on-time, and under-budget.
Split into simpler sentences: The decision was based not only on aircraft performance but also on contractor performance. During the U- 2 project, Lockheed had proven its ability to design advanced aircraft in secret, on-time, and under-budget.