Considering the explosiveness of its subject matter, "Enola Gay," the three-hour NBC Big Event tomorrow at 8 on Channel 4, should have been a far more substantial production. The story of the decision to drop the atom bomb on Hiroshima and preparations for the mission is put over with little more impact than the typical war-caper drama.

The producers harvested a bumper crop of second bananas to play soldier in the film; most of the actors are tired and callow TV series stars like the insufferable Gregory Harrison ("Trapper John, M.D."), the simpering Gary Rank ("Family"), the abrasive Robert Walden ("Lou Grant") and, as bombardier Jake Beser, the minimal Billy Crystal ("Soap"). Armageddon deserves a better class of pallbearer than this.

The screenplay intercuts scenes of the top-secret training sessions, meetings in Washington over whether the bomb should be used, and dainty little vignettes of life in Japan, where James Shigeta, as Field Marshall Abehata, is seen leading a strangely leisurely life devoted to walking in the woods and spouting philosophies.

An attempt at humanizing the tale with the domestic strife endured by Col. Paul Tibbets, pilot for the mission (played prissily by Patrick Duffy of "Dallas") is an awkward bungle, but it does bring back to the screen a taller, leaner and older Kim Darby, who does what she can with the role of Tibets' wife. According to the screenplay, she announces she's leaving him the night he leaves for Tinian Island; you half-expect her say, "What'll it be -- me or the bomb?"

At three hours, the film is far, far too long, and it's so eager not to offend anyone with questions about the propriety of Hiroshima that it opens with an elaborate prologue (to the men who "bravely did their duty and, in their innocence, changed all of human history") and goes very sparingly on scenes of the destruction the bomb wrought.

Yet it cannot be denied that when the propellers on that B29 roar on for the flight to Japan, the pulse quickens and the heart sinks. And if one point of the film is that the men who undertook this mission were ordinary, the mass anemia of the cast may be somewhat appropriate, though it is dramatically indefensible.

The progress of the war is told through newsreel footage, not always in the right chronology. It is made to appear that the battle for Naha on Okinawa took place prior to Christmas of 1944, when it actually occurred the following April, near the day FDR died. Not very authentically, Crystal says in modern hip talk, "I'm, like, insignificant here," which doesn't sound very 1940s to me.

When the bomb goes off, the footage of the mushroom cloud is obviously from one of the desert tests. Although subsequent aerial shots of devastation are fittingly grim, the bombing doesn't have the impact Sidney Lumet got out of an imagined attack on New York City in the movie "Fail Safe," when he combined various glimpses of city street life and froze them in suspension at the moment of detonation.

"A warning followed by a dud would be a real disaster," says one of the characters in "Enola Gay." The film is no disaster, but it's more of a dud than a warning.