Eastern Airlines has formalized its prior bid to acquire National Airlines by offering to enter into a merger agreement almost identical with the pact between Pan American World Airways and National.

The major difference is that Eastern has offered $50 a share for National stock, compared with the $41 a share Pan Am has tendered.

Texas International Airlines also is attempting to take over National but since National spurned TXI's over-tures, the effort is considered a hostile takeover attempt.

Eastern said yesterday it has already obtained an acceptable loan commitment from Chase Manhattan Bank to provide more than $100 million of the $425 million that would be necessary to consummate the merger. The airline also said it expected to obtain commitments for the rest of the money by Jan. 15, as well as the needed lenders' consents and approvals by Feb. 1.

The Eastern offer is subject to execution of the merger agreement by National, approval of National's shareholders, and approval of the Civil Aeronautics Board and President Carter, the latter because the merger would involve in ternational routes.

Pan Am has said previously it had funds available through various bank arrangements to finance its bids.

Par Am and TXI each have about 23 percent of National's stock, and have CAB permission to increase their holdings to 25 percent if it is insulated in a trust barring the exercise of control over National.

Eastern entered the bidding for National earlier this month, a good month and a half after CAB hearings had begun on the other two applications for merger approval.

Last week, the board refused to consolidate Eastern's application into the same hearings, and assigned a separate administrative law judge to hear the Eastern application beginning in January, However, the board said it will wait until those hearings are completed before making a final decision on National's fate. The board had hoped to make its decision in March.