Pan American World Airways announced yesterday that it had obtained enough National Airlines stock to give it majority ownership.

Pan Am said that at the close of business Wednesday, it had obtained a total of 4,398,500 shares of National, equal to 51.4 percent of its outstanding common stock. The total purchase price for Pan Am's stake in National was $186,057,257.50 for an average price per share of about $42.30.

The price paid for the last shares it bought is believed to be very close to the $50 it has agreed to pay National stockholders for each share under the merger agreement it has signed with National.

Despite Pan Am's majority stock position, the flag carrier still has some hurdles before getting actual control of National. The stock it has acquired still has to be placed in a nonvoting trust which bars it from exerting any control over National until merger proposal is approved by a final Civil Aeronautics Board order and by President Carter.

The CAB two weeks ago tentatively approved the competing bids of Pan Am and Texas International for control of National, and still has pending a third decision on Eastern Airline's competing proposal to merge with National.Although Eastern's chances are believed to be slim, officials of Eastern - which holds 100 shares of National - remained "optimistic" yesterday that regulatory approval would be given.

Texas International officials have said the CAB's tentative approval of both its and Pan Am's bids put the Houston-based carrier at a distinct disadvantage since National share-holders had approved the Pan Am-National merger and are bound by contract to go through with it.

At the time the CAB made its tentative decision on the two bids, Pan Am had a 24.1 percent interest in National while Texas International's stake was slightly higher.

Meanwhile, heavy trading in the stock of Continental Airlines over the last few days has led to speculation that it might be a possible takeover target.

Continental Airlines was the most active Big Board stock yesterday, on volume of 652,000 shares. It closed at 13 3/4, up to 15/8; the stock closed at 9 last Friday, the day before the CAB tentatively rejected its proposed merger with Western Airlines. On Wednesday, Continental stock was up 2 1/8 in heavy trading.

Bruce Plowman, a spokesman for Continental, said the Los Angeles-based airline hasn't had any discussions with or even inquiries from any firm about a merger offer or the possible acquisition of Continental, and didn't know who was buying its stock.

Rumors were rampant on Wall Street, but speculation that buyers of stock might be Eastern or Braniff Airlines were denied by both.

Speculation also centered on Texas International which will make nearly $40 million should it sell its National shares at $50 apiece (it paid an average of $28.20 for its 2.1 million shares).

Officials of Texas International were difficult to reach yesterday because of widespread flooding in Houston, its headquaters city. A source close to the airline said that as far as he knew, Texas International was not the purchaser. Any buyer of stock who acquires 5 percent or more in a firm must report such an acquisition to the Securities and Exchange Commission, so if a single buyer was doing the purchasing, it will be known soon, lawyers here said.

Continental is considered a good buy, according to airline observers. The asset value of its fleet of air-planes makes the stock price extremely undervalued, they say. Continental has just $200 million in long-term debt while its fleet of 67 air-planes is worth an estimated $750 million.