Texas International Airlines said yesterday it has acquired 18.2 percent of the stock of National Airlines in its bid to take over the larger company.

In filings with the Securities and Exchange Commission, Texas International disclosed that it owned $1,553,300 shares of National common stock as of the close of business Tuesday, almost a doubling of its holdings since it announced its acquisition intentions last month.

TXI said it has paid a total of $42,2 million for the stock for an average price per share of about $27,16. National closed at 34 1/4, down, 3 5-8, on the New York Stock Exchange yesterday in consolidated trading.

A spokesman for Pan American World Airways, which also is seeking a merger with National and offered $35 a share in that quest, said no disclosures of additional stock purchases have been made above the 4.8 percent it previously announced buying.

Under SEC disclosure requirements, the companies are required to report any incremental stock purchases that increase their ownership by 1 percent.

The Civil Aeronautics Board has given TXI and Pan Am each temporary authority to purchase up to 25 percent of National's stock if the additional stock acquired is placed in trusts that prevent the exercise of voting control of National.

However, both airline could be forced to divert themselves of the stock bought under this temporary authority if the CAB decides after a public-comment period that the trust arrangements do not adequately insulate the two acquiring airlines from control of National during the board's consideration of their merger applications.

TXI also announced yesterday that a subsidiary had completed the overseas sale of $25 million of 7.5 percent convertible subordinated debentures due in 1993, with the revenues to go into general corporate funds.

In New York, Pan Am announced that it has decided to call for redemption the balance of its 10.5 percent convertible-subordinated debentures, due in 2001. Last month, Pan Am called in $25 million worth of the issue. Pan Am said $69 million worth of the debentures were outstanding as of the close of business Monday.

By calling in the debentures, Pan Am expects to strengthen its balance sheet by reducing its debt and increasing equity. The airline also will save a considerable amount of money it would have had to pay in interest on the bonds. However, floating more stock into the capital structure - bondholders electing to convert their debentures into shares get a better deal than the redemption price offers if Pan Am's stock price stays up - there will be some dilution in Pan Am's earnings per share.