Complex sentence: By the outbreak of World War II, seven squadrons were operational with the Whitley, with the majority flying Whitley IIIs or IVs, with the Whitley V having just been introduced.
Split into simpler sentences: By the outbreak of World War II, seven squadrons were operational with the Whitley. The majority were flying Whitley IIIs or IVs as the Whitley V had only just been introduced.