As "Arms and the Girl" opens, a cannonball crashes through the back wall of a barn, lands on the stage, and rolls directly toward an alarmed audience. The new Theatre Guild musical comedy is aimed at a target just as broad--the old New England custom of bundling--and it scores a hit almost as solid as the one by the cannonball.

Bundling is the subject of "The Pursuit of Happiness," by Lawrence Langner and Armina Marshall, and it is on this play that "Arms and the Girl" is based. The new product has a rowdy, bumptious book by Herbert and Dorothy Fields and Rouben Mamoulian. It calls on Nannette Fabray to play a girl who tries to win the American Revolution singlehanded, but succeeds only in causing complete confusion. Her mission brings her to make clever use of her bed in times of crisis--with and without a large, saw-toothed "bundling board." Miss Fabray has plenty of assurance and ability as a performer. She lacks only the all-consuming vitality and sense of timing that her part calls for.

Bundling is a coeducational operation. The man in this instance is Georges Guetary of the French stage. In the part of a runaway Hessian who is at first reluctant about bundling, he proves that he has an excellent voice, genuine comic ability and a charming stage personality. John Conte is a consistently amusing colonel without any qualms about accepting the bundling custom.

Admirers of Pearl Bailey will not be surprised to hear that she stops the show with two of Morton Gould's tunes, "Nothin' for Nothin'" and "There Must Be Something Better Than Love." She talks, moves, sings, and dances as if she were perpetually tipsy. It's always a pleasure to watch her perform, because she makes even commonplace material come alive.

The two musical numbers already noted, and "That's My Fella," comprise the best of Gould's current crop of songs. Others in the show don't measure up to them simply because they're not "humable." Michael Kidd's first act dances are witty, clever, and completely refreshing creations, and it's a pity there are no more of them. The sets by Horace Armistead have all the briskness of an American primitive. They stand up well under the mock battles, chases, and explosions that occur on stage.

The first act moves at a fast pace, thanks to Rouben Mamoulian's direction. The show suffers from a slight ease of second act sag, but in spite of this, a bouncing, healthy show was born Tuesday night at the Shubert.