Original Dungeons & Dragons (1974):
Fire Ball: A missile which springs from the finger of the Magic-User. It explodes with a burst radius of 2" (slightly larger than specified in CHAINMAIL). In a confined space the Fire Ball will generally conform to the shape of the space (elongate or whatever). The damage caused by the missile will be in proportion to the level of its user. A 6th level Magic-User throws a 6-die missile, a 7th a 7-die missile, and so on. (Note that Fire Balls from Scrolls (see Volume II) and Wand are 6-die missiles and those from Staves are 8-die missiles. Duration: 1 turn. Range: 24"
Holmes D&D (1977):
Wand of Fire Balls — On activation, the wand produces a fire ball which will travel any distance, up to 240 feet, desired by the user and then explode with a burst radius of 20 feet, doing 6 dice of damage to anyone within range who fails their saving throw (half damage if saving throw is made). Fire ball blasts in confined spaces generally conform to the shape of the space (so watch out!).
Advanced Dungeons & Dragons (1978):
A fireball is an explosive burst of flame, which detonates with a low roar, and delivers damage proportionate to the level of the magic-user who cast it, i.e. 1 six-sided die (d6) for each level of experience of the spell caster. Exception: Magic fireball wands deliver 6 die fireballs (6d6), magic staves with this capability deliver 8 die fireballs, and scroll spells of this type deliver a fireball of from 5 to 10 dice (d6 + 4) of damage. The burst of the fireball does not expend a considerable amount of pressure, and the burst will generally conform to the shape of the area in which it occurs, thus covering an area equal to its normal spherical volume. [The area which is covered by the fireball is a total volume of roughly 33,000 cubic feet (or yards)]. Besides causing damage to creatures, the fireball ignites all combustible materials within its burst radius, and the heat of the fireball will melt soft metals such as gold, copper, silver, etc. Items exposed to the spell's effects must be rolled for to determine if they are affected. Items with a creature which makes its saving throw are considered as unaffected. The magic-user points his or her finger and speaks the range (distance and height) at which the fireball is to burst. A streak flashes from the pointing digit and, unless it impacts upon a material body prior to attaining the prescribed range, flowers into the fireball. If creatures fail their saving throws, they all take full hit point damage from the blast. Those who make saving throws manage to dodge, fall flat or roll aside, taking 1/2 the full hit point damage - each and every one within the blast area. The material component of this spell is a tiny ball composed of bat guano and sulphur.
B/X D&D (1981):
This spell creates a missile of fire that bursts into a ball of fire 40' across (20' radius) when it strikes a target. The fire ball will do 1-6 (1d6) points of fire damage per level of the caster to each creature within the sphere of fire. EXAMPLE: A fire ball cast by a 6th level magic-user will burst for 6-36 (6d6) points of damage. If the victim of the fire ball saves vs. Spells, the spell will only do half damage.
BECMI D&D (1983):
This spell creates a missile of fire which explodes into a ball of fire of 20' radius when it reaches the desired range or strikes a target. The Fire Ball inflicts 1-6 (1d6) points of fire damage for each level of the spell caster. Each victim within the area of effect takes full damage unless a Saving Throw vs. Spells is made. Even if the Saving Throw is successful, the victims take half the rolled damage. For example, a Fire Ball cast by a 6th level magic-user explodes for 6-36 (6d6) points of damage. If the total roll is 24, all within the area who make their Saving Throws take 12 points of fire damage.
AD&D 2nd edition (1989):
A fireball is an explosive burst of flame, which detonates with a low roar and delivers damage proportional to the level of the wizard who cast it - ld6 points of damage for each level of experience of the spellcaster (up to a maximum of 0d6). The burst of the fireball creates little pressure and generally conforms to the shape of the area in which it occurs. The fireball fills an area equal to its normal spherical volume (roughly 33,000 cubic feet - thirty-three 10' x 10' x 10' cubes). Besides causing damage to creatures the fireball ignites all combustible materials within its burst radius, and the heat of the fireball melts soft metals such as gold, copper, silver, etc. Exposed items require saving throws vs. magical fire to determine if they are affected, but items in the possession of a creature that rolls a successful saving throw are unaffected by the fireball.
The wizard points his finger and speaks the range (distance and height) at which the fireball is to burst. A streak flashes from the pointing digit and, unless it impacts upon a material body or solid barrier prior to attaining the prescribed range, blossoms into the fireball (an early impact results in an early detonation). Creatures failing their saving throws each suffer full damage from the blast. Those who roll successful saving throws manage to dodge, fall flat, or roll aside, each receiving half damage (the DM rolls the damage and each affected creature suffers either full damage or half damage [round fractions down], depending on whether the creature saved or not).
Holmes basic D&D is the only version that specifies the initial missile departing from the spell-caster takes the form of a fire-ball itself, rather than a "missile" or "streak" that "explodes" or "flowers" or "bursts" or "blossoms" into a fireball when reaching its destination (or on impact).
Artist's depiction of a fireball before it strikes, from adventure module G3 Hall of the Fire Giant King, 1978