sentences:  Over France in World War I, where man and flying machines were first forged into a formidable fighting unit.At the start of the war, the adversaries could muster only a few hundred frail aircraft between them.Yet, in less than 4 years, Britain had commissioned an independent Air Force, had built over 50,000 aircraft and trained 26,000 pilots for observation, strafing, bombing and whatever was necessary to protect its air space.