THE Boeing Company - maker of 60 percent of all commercial aircraft sold so far this year - is bracing for perhaps its most serious competitive threat in recent memory.Airbus Industrie, the government-owned European plane consortium - has moved aggressively to match Boeing plane for plane, both in terms of offering a diversity of aircraft models to choose from, and in outright sales. Moreover, Airbus, armed with government subsidies, is targeting many of Boeing's best customers. Airbus has already supplanted McDonnell Douglas as the world's second-largest maker of commercial aircraft. It has been particularly successful selling planes at the small-size end of the market. Now, Airbus is challenging Boeing's most profitable sales area: wide-bodied jets. At the Paris Air Show earlier this year, Airbus stunned the aviation world by announcing that it was considering building a 600- to 700-seat plane - larger than Boeing's profitable 747. The Boeing-Airbus competition has more at stake than just corporate market shares and executive egos. Airplanes constitute the United States' No. 1 export product. In 1990, US jet aircraft and aerospace sales abroad totaled $31 billion, compared with imports of $6 billion, notes Sam Nakagama of Nakagama & Wallace Inc., a New York economic consulting firm. That means a surplus of $24 billion. Computers, America's No. 2 export, produce little net surplus for the US. In terms of the Airbus competition, Boeing "is not in as bad a position a McDonnell Douglas," which is down to just a few key models, says Mr. Nakagama. Still, Boeing faces a major challenge, he says. Given the costs involved in developing a new aircraft, such as a 700-seater, "Boeing is going to need important allies, both financially and commercially." Airbus "is a tough competitor," says Dean Thornton, president of Boeing Commercial Airplane Group at his corporate offices just south of Seattle. But, he quickly adds, the Seattle company is determined to maintain its aviation primacy. "This business has always been an international sales business; 60 percent of our sales are outside the United States." He says the super-jumbo jet decision is crucial, since such a plane would be important to Pacific and Atlantic international carriers. Mr. Thornton notes that Boeing has three major options regarding a wide-bodied aircraft: * It can build an entirely new plane. But costs would be huge. * It can stretch out the existing 747-400, which would add "between 50 and 100 seats." * It can build a double-decker 747, with an upper floor. Boeing's dilemma is that the company is currently preoccupied with developing its new twin-jet 777; the first 777, which is being built in a partnership with three Japanese manufacturers, is expected to be delivered to United Airlines in 1995. But, says Thornton, that means that Boeing would have a window of opportunity in the mid-to-late 1990s to develop a new superplane. Boeing officials decline to identify how much the 777 is costing, but outside estimates run around $1.5 billion to $2 billion. Boeing reportedly has cash holdings of $3 billion, down from about $3.3 billion a year ago. Moreover, the company is also part of a group of aircraft manufacturers studying possible development of a new supersonic commercial transport. But even if Boeing's contribution were to be just part of a larger group effort, the development costs would presumably be substantial. Boeing, meantime, is seeking legal action against Airbus because of the European subsidies; Boeing's legal action is being brought through the courts, as well as the General Agreement on Tariffs and Trade, according to Lawrence Clarkson, senior vice president of the commercial group. "Boeing is not going to cede its markets to anyone, including Airbus," says Lawrence Harris, an aircraft analyst with Kemper Securities Group. He notes that many of Boeing's best customers - Lufthansa, British Airways, and Air France - are found right within the host nations of the Airbus.