McDonnell Douglas Corp. yesterday announced a 36 percent increase in first-quarter earnings from the previous year, while Phillips Petroleum Co. said its quarterly earnings fell 45.1 percent.

McDonnell Douglas said it earned $91.7 million ($2.28 per share) on sales of $2.7 billion in the first three months of 1985.

The results include a one-time gain of $10 million from the sale of land in El Segundo, Calif., the St. Louis-based corporation said.

First-quarter 1984 earnings were $67.5 million ($1.69) on revenue of $2.03 billion.

McDonnell Douglas said its combat aircraft and transport aircraft segments both had higher earnings in the first quarter of 1985 compared with the same period a year ago. Phillips Petroleum Co., citing expenses related to its survival of two hostile takeover fights, said first-quarter earnings fell to $106 million (71 cents a share) from $193 million ($1.26) in the same period a year earlier.

The nation's eighth-largest oil company, based in Bartlesville, Okla., said revenue edged up 1.5 percent, however, to $4.02 billion from $3.96 billion. Weirton Steel Corp. posted first-quarter earnings of $10.2 million compared with $9.7 million for the same period in 1984, Chairman and President Robert L. Loughhead announced.

Sales for Weirton, an employe-owned company, were $295.6 million, compared with $283.1 million in 1984's first quarter. The company shipped 592,403 tons compared with 572,731 tons in the first three months of last year.

In 1985, Weirton plans about $65 million in capital expenditures in order to refurbish primary and finishing facilities and continue with progress on environmental programs.