United Airlines placed at $1.2 billion order yesterday for 30 Boeing 767 widebodied jets, a drawing-board plane that will be the first completely new plane built by an American manufacturer since 1966.

United's is the first solid order for the 767 and assures that the Boeing Co. will launch production of the new generation aircraft. "The confidence shown by the size of their order gives us the encouragement to proceed into production as aggressively as we know how," Boeing Chairman T.A. Wilson said yesterday.

The new jets - powered by two Pratt & Whitney Aircraft engines each - are scheduled for delivery between 1982 and 1984. The planes, with a 2,200-mile range, will seat 197 passengers seven abreast - two-three-two across with two aisles.

United also placed an order for 30 more Boeing 727-200s, powered by three Pratt & Whitney engines each to be delivered by 1980. The new order is in addition to the 46 ordered in 1976 and 1977. Price tag for the new order of the 132-seat planes is $400 million.

United chose the new Boeing 767 over the Airbus A300-B10 offered by Airbus Industrie, a European - primarily French-German - consortium. Airbus received several orders from European airlines in the past few weeks for the new B10, a smaller version of the A300-B4 that has been purchased by Eastern Airlines.

United officials has also considered the possibility of doing without a new generation airplane and ordering still more 727-200s or the larger McDonnall-Douglas DC-10s.

Richard J. Ferris, United's president and chief executive officer, said the carrier chose the 767 after a year long technical and economic analysis indicated that it "fills the bill the best" for the "mission" the plane was to perform over United's route structure. "It came out in our evaluation overall to be a better economic decision for United," he said.

Although he expressed pleasure that United's order would mean "good things" for U.S. employment and the balance of payments, and the aircraft industry, he said the fact that the Airbus was produced abroad was not a factor in the airline's decision. "We did not consider national origin or origin source," he told a press briefing. If Airbus had come out first in United's evaluation - "if the numbers were different" - United would have purchased it, he maintained.

United's economic evaluation concentrated on price, operating performance and the financing package, Ferris said, with operating performance carrying three times the weight of price when calculating return on invested capital.

Ferris and other officials declined to break down specifically what factors made the 767 the favorite.

United announced its decision after a joint meeting of the boards of directors of both United Airlines and UAL, Inc., the parent company. The decision to buy the 767 was unanimous, a United spokesman said.

Ferris said a "substantial" portion of the $1.6 million committed yesterday would come from internally generated funds but that United's lenders would be approached for some of it. At the end of last year, United had $679 million on hand in cash and investments, an amount to which it surely has added this year.

The entire order of new planes will be used to replace older, noisier, less efficient aircraft. Ferris noted yesterday that the 767 would be nearly 35 percent more fuel efficient per seat mile than the planes they will replace.The 767 will add some capacity as well as the 197-seater will be replacing planes with an average of 117 seats.

In a somewhat related development yesterday, the Treasury Department said the Carter administration will seek an international agreement this fall to limit competition in export credit financing of commercial aircraft.

Treasury official Gary Hufbauer told a House Ways and Means Sub-committee that the U.S. is concerned about some recent financing arrangements involving sales by Europeans to U.S. airlines.

He said the U.S. isn't trying to keep the Airbus or other foreign-built airplanes out of the market. But he said it appears that both the British Government, in financing arragements for Rolls-Royce engines for Lockheed £1011s, and the West German and French governments, in financing a sale of 23 Airbus planes to Eastern, have gone beyond an international understanding on export credit terms of commercial aircraft.