A Midwest Express jetliner crashed into a field just after taking off from Milwaukee yesterday afternoon, killing all 31 people aboard.

Seconds before the crash, the pilot radioed the air traffic control tower at Milwaukee's General Mitchell Field that "I have an emergency," but he did not describe it further, Federal Aviation Administration officials said.

Witnesses said the plane climbed to 600 or 700 feet on what appeared to be a routine takeoff in clear weather. Then one of the two engines emitted a puff of smoke and may have burst into flames, a witness said.

The plane rolled to one side, then crashed nose first in a heavily wooded area south of the airport.

Pamela Murr, a traffic reporter for WTMJ radio, said the plane made "a couple of barrel rolls and went down nose first. It burst into flames."

"There is nothing we could have done if we had been there right on the scene," Richard Seelen, assistant Milwaukee fire chief, told the Associated Press. "It was total devastation."

The crash of Midwest Express Flight 105, from Madison, Wis., to Milwaukee to Atlanta, came at about 3:25 p.m. CDT. It was the third major plane crash in the United States this year and at least the eighth major accident worldwide in what was already the worst year in civil aviation history. At least 1,400 people have died in 16 accidents.

Midwest Express is a three-plane airline headquartered in Appleton, Wis., and owned by Kimberly-Clark Corp., best known for paper products. It operates between Milwaukee and Dallas-Fort Worth, Atlanta and Boston as well as serving Appleton and Madison. Midwest Express has been in business since June 1984, but yesterday was only the fourth day of its Madison-Milwaukee operations, airline president Timothy Hoeksema told reporters.

Airline spokesman Jose Oller said 31 people were aboard the plane. He said most passengers got on in Milwaukee and he believed 10 were employes of Kimberly-Clark.

The passenger list was being withheld last night.

The aircraft, a twin-engine McDonnell Douglas DC9, is a mainstay of the short-haul U.S. fleet and almost 1,000 of them have been sold worldwide.

This version, a DC9-10, was the 393rd DC9 manufactured by the Douglas Aircraft Co. in Long Beach and was delivered new in October 1968 to a Venezuelan carrier. It could seat up to 80 passengers.

The DC9, like all major commercial jetliners, has an outstanding safety record. The most recent DC9 accidents involving fatalities occurred in 1983. In June, an Air Canada DC9 caught fire in the air and made an emergency landing at Greater Cincinnati Airport; 23 aboard survived but 23 others were unable to escape. In December, there were 92 fatalities in a runway collision in Madrid involving a DC9 and a Boeing 727.

The DC9 is powered by two Pratt & Whitney JT8D engines, mounted high on the fuselage in the rear. That is the same basic engine involved in the Aug. 22 British Airtours accident in Manchester, England, in which 55 people were killed on a Boeing 737. David Long, a spokesman for Pratt & Whitney, said last night that the Midwest Express plane's engines were JT8D-7s, an earlier model than the JT8D-15s on the British Airtours plane.

At least two witnesses told Milwaukee reporters that they "heard a pop" before the plane started to roll. One of the witnesses, construction worker Russ Lewandoski, said the plane apparently had an engine problem.

"All of a sudden he fluttered," Lewandoski said. "He lowered the nose real good. Then he banked to the right. It seemed like he turned to the engine that was dead, then it went down."

There was too little information last night to permit informed speculation as to the cause. The failure of a single engine on a twin-engine jet would not normally create an uncontrollable or unflyable condition unless something else went wrong at the same time.

However, if one of the engines did catch fire as some witnesses reported, a possible cause would be a disintegrating engine turbine blade or disc. Several such events have been reported in the history of JT8D, and a flying disc could sever vital control cables.

Control difficulties, which the pilot obviously encountered, can stem from a myriad of problems, including hydraulic failure or structural failure of a major control component, such as the tail.

FAA observers in the tower saw the plane lift off the end of Runway 19 Right, the main airline runway, which is 9,600 feet long. Then they saw it drop behind the tree line. "The next thing the tower saw was smoke and fire from a large ball of fire," FAA's Chicago spokesman Mort Edelstein said.

Charles Hopkins, spokesman for the suburban Oak Creek police department, said attempts to search the area for victims were impeded by the density of woods and by traffic congestion on nearby streets.

Catherine Pokorny, nursing supervisor at Trinity Memorial Hospital near the airport, said, "Even before we were put on alert, people and staff from the hospital were returning to work because we're closest to the airport. They could see the fire, it was so high." But when they realized there were no survivors, off-duty personnel departed.

Scott Scrima, 27, of Waukesha, employed at an airport freight company, said he walked into the woods and found smoking debris scattered over about a half acre.

"I saw a lot of smoke," Scrima said. "I saw a lot of little pieces. I didn't see anything big. It looked like a forest fire" had swept the scene.

A National Transportation Safety Board team headed by Chairman Jim Burnett was en route to the crash site last night to begin its investigation. Officials on the scene said the plane's flight recorders, the so-called "black boxes" that record pilot conversations and airplane peformance characteristics, had already been recovered.