NY -- Standard&Poor's CreditWire 12/16/96 -- Standard&Poor's today has placed its ratings of Boeing Co. and McDonnell Douglas Corp. on CreditWatch with positive implications (as listed below), following the announcement that the two companies have signed a definitive agreement whereby McDonnell Douglas will merge with Boeing in a stock transaction valued at about $13.3 billion. Boeing will be the surviving entity. Ratings on McDonnell Douglas Finance Corp. are also placed on CreditWatch (see list), since those ratings are closely tied to those of the parent. The merger would create the world's largest aerospace firm, with 1997 revenues estimated at $48 billion. The combination would enhance Boeing's competitive business position as a leading player in defense and