[information coming whenever]
At the beginning of the 21st Century, two military groups known as the Regular Army and the Rebellion are at war. The Regular Army is a government- run military force used for various tasks such as peacekeeping and fending off terrorism. The Rebellion is formed by a group of people whose wishes are to change the world-government into a military controlled one.
In 2026, the Rebellion Army launches an assault that pushes the Regular Army forces into the brink of destruction. No one expected this except for the Rebels who came up with these plans. The fault was mostly in the hands of the Regular Army commanding officers, who failed to realize the value of information that the Regular Army intelligence division had provided.
With superior numbers of troops and weapons, the Rebels overwhelmed the Regular Army in all aspects. Seeing their disadvantage to carry out direct attacks against the Rebellion, the Regular Army then decided to carry out numerous special operations and create certain vehicles to accompany their commandos. Not long after, a newly designed tank, code named “Metal Slug”, was forced into production.
The war went on for two years. During those two years, the Regular Army has been stockpiling the “Metal Slug” tanks in their caches with hopes to launch a massive assault against the Rebels and end the war. Things went well until 2028. The Rebellion forces found out and captured many of the caches, along with many of the tanks in them intact. Without those tanks the Regular Army could not even hope to win. And with those tanks under the Rebel control, it was just a matter of time until their demise.
With the government and the military in shambles, Lieutenant Rossi gathers up most of the scattered Regular Army troops he can muster to launch a desperate counterattack. Their mission objective: recapture the stolen vehicles and use them to destroy the opposition. If the vehicles can’t be captured, then destroy every last one of them.
from Metal Slug Anthology manual
The development history of the first Metal Slug is probably the most documented of the series, not just from the sheer amount of concept art, but from a series of videos by Retro Game Test: Metal Slug History Part 1 and Part 2. Credit goes to Youloute (ganked from Gaming Hell), Glowsquid (from NeoGAF posts), and Divison 六 for translations.
Nazca Corporation was formed from members of Irem's then-defunct arcade staff and served as a subsidiary to SNK. Their job was to port Neo Geo games to PlayStation and SEGA Saturn, but demanded to make an arcade game of their own; SNK allowed them to make it in their off-time.
"We thought having normal player characters would be boring, and we wracked our brains for awhile to come up with something different. That was when we hit upon the idea for the tanks. Then, once we made them jump, crouch, and move around, they seemed like living creatures, and we thought this would grab players’ attention."
Meeher, 1999 development interview
The game was originally played only with the Metal Slug; there was no on-foot gameplay (though human pilots, engineer Phil Gene and Michiko Nakajima, were drawn up before Marco and Tarma were conceived - see Concept Art). With the tank as the central focus, the game was to be a lot more slower-paced, focusing on carefully controlling your sluggish vehicle across short levels that were thick with combat, requiring cautious play. All weapons would fire simultaneously upon a button press, and cannon shells were in infinite number. The crew briefly experimented with a button-based aiming system for the vulcan cannon, though it evidently didn't last long.
The P.O.W.s were originally Regular Army soldiers (coloured in blue, but would change colour to match the player) who, when rescued, would tag along and throw grenades, climb on top to fire a bazooka, or even board a mounted turret if one was handy; Early Footage shows this feature in more depth. Only two soldiers could support the player at a time, but any additional rescued soldiers would be stored as stock, and automatically tag in to replace a casualty. Their additional firepower and option-like behaviour appeared to be a heavy focus in the early level designs.
The visual design was partially inspired by the works of Hayao Miyazaki (particularly his Daydream Data Notes illustrations), though the tone of the game was considered more dark at this point, with little to no humour outside of the characterful soldiers. The stages and overall game length were shorter, with less complex stage designs to account for the Metal Slug's limited movement capabilities. Mission 1 took place almost entirely in dark swamp, and Mission 3 only covered the Rebel base and forest, for instance. Mission 2 was not developed at this point, but the rest of the game appeared visually identical to the finished product.
"In order to create and release a game, tons of ideas or designs are born and destroyed.
Actually, in the first title, there was a stage that we had to redraw almost half of the background graphic because it ended up not matching our direction as development progressed."
from Metal Slug Anthology interview
After location testing in Osaka in 1995, the game received a significant overhaul. Marco and Tarma were introduced as the player characters, the soldier P.O.W.s were replaced with the iconic unshaven ragamuffins, and the game's length was extended by SNK's demand to make it more attractive to Neo Geo AES home cartridge buyers. The music was sped up as the "bide your time" gameplay was out the window by this point, though since the soundtrack already been composed, Missions 2 and 3 had to reuse the same track.
With human characters, more platform game elements were incorporated into the level design; Mission 2 was built from the ground-up for this version, along with the first half of Mission 3. Huge chunks of Mission 1 and 3 were also redrawn; all levels received modifications to enemy layouts to account for the new play style, though Missions 4 and 5 were mostly left structurally unaltered.
There are a few unused sound effects in the game's data that are possible leftovers from its early development; credit goes to Ivo for digging these up!
Various voice clips from the announcer. Would these be shouted by the support soldiers you picked up along the way? Other unused sound effects include the merchants' screaming from Metal Slug 2 and two clips of a campfire burning.
Using Nebula Jukebox will often find snippets of music among the sound effects, usually of a single-channel played sped-up, like this rendition of the Mission 2 track. I don't know enough about audio to know if they're genuine unused tracks or just the sound data being read in strange ways, but bits of that track show up a few times throughout the data.
Credits (Neo Geo)
HIYA! (Takushi Hiyamuta / 冷牟田 卓志)
KIRE-NAG (Kazuma Kujo / 九条一馬)
A.KUROOKA (Atsushi Kurooka 黒岡 厚至 )
Credits (SEGA Saturn)
METAL SLUG for SS STAFF
SPECIAL THANKS TO
KENSHI NARUSE (成瀬憲史)
富永 みーな (Miina Tominaga)
株式会社 夢工房 (Yumekobo Co. Ltd.)
SNK第４企画部ALL-STAFF (SNK 4th Planning Division)