Complex sentence: Bell offered a more robust version of the OH-58 in their model 406 aircraft, and Hughes offered an upgraded version of the OH-6, and on 21 September 1981, Bell Helicopter Textron was awarded a development contract.
Split into simpler sentences: Bell offered a more robust version of the OH-58 in their model 406 aircraft, and Hughes offered an upgraded version of the OH-6. On 21 September 1981, Bell Helicopter Textron was awarded a development contract.