McDonnell Douglas Corp. announced earnings of $51.2 million or $1.33 per share on revenues of $3.242 billion in the first quarter of 1988. Earnings in the same quarter last year were $43.0 million or $1.06 per share on revenues of $3.036 billion.

The company said transport aircraft revenues were sharply higher as a result of 24 MD-80 deliveries in the first quarter of 1988 as compared to 13 in the first quarter of 1987. Revenues in other major lines of business grew only slightly.McDonnell said non-recurring items had a substantial effect on MDC's results in both the first quarter of this year and the first quarter of last year.

A spokesman said several factors have combined to put profit margins under pressure across most lines of business. These include substantial investment in development of the MD-11 tri-jet and in competing for the U.S. Air Force's Advanced Tactical Fighter program; cost/production problems encountered in a number of existing programs, including the MD-80, the F-15 and the F/A-18; and higher interest costs from increased average borrowings.

Firm backlog on March 31, 1988, was $22.048 billion, compared with $17.577 billion a year earlier. Employment on March 31 was 114,155, compared with 105,682 a year earlier.