Complex sentence: The aircraft, dubbed 500P (penetrator) by Hughes, began as an ARPA project, codenamed "Mainstreet," in 1968; development included test and training flights at Area 51 in 1971.
Split into simpler sentences: The aircraft, dubbed 500P (penetrator) by Hughes, began as an ARPA project, codenamed "Mainstreet", in 1968. Development included test and training flights at Area 51 in 1971.