For the last five months, this new-generation stealth fighter jet has reportedly been undergoing secret tests. Behind closed hangars and under strict security, engineers from Boeing have been working on what could become the most lethal warplane the world has ever seen. For the last five months, this new-generation stealth fighter jet has reportedly been undergoing secret tests. Behind closed hangars and under strict security, engineers from Boeing have been working on what could become the most lethal warplane the world has ever seen.