Answer: It is "The first successful airplane was invented in 1903, the first successful airplane stayed in flight for a few seconds."
Explanation:
This is because we need to show and explain what the topic of this sentence is about, without it, it seems to mess up how the paragraph is flowed, if we said that "The first successful airplane stayed in flight for a few seconds, The first successful airplane was invented in 1903." It seems to explain what happened and then it seemed to just say what this is, giving no point of what the first part was even about, although it can work in other scenarios when the first part is a teaser of what will happen in a story, it would reappear later on in the story or whatever it may be.
